First off, I'm also pretty skeptical regarding Santa Claus and Leprechauns. I'd like to have some positive evidence for believing in something, particularly something contrary to all my experience with how the world works. As Carl Sagan said, "Extraordinary claims require extraordinary evidence." But even beyond that, I find the standard monotheistic conception of 'God' to be incoherent and internally contradictory.
My primary reason is a fairly common one - the "Argument from Evil", or the "Problem of Evil". If God is omnipotent, omniscient, and omnibenevolent... then whence evil? How could evil arise against the wishes of such a being? (Note: some theists assert things like "...it is impossible to distinguish evil from good unless one has an infinite reference point which is absolutely good." I think this is incorrect, but even so, note that the following argument does not depend on a specific definition of "Evil". It merely requires that God desires certain outcomes that end up not happening.)
Below I detail the argument in the most general possible terms. Note that I don't regard this as an ironclad logical proof (nothing outside of mathematics is really susceptible to that) but I do think, based on this reasoning, that the odds of something like the traditional monotheistic 'God' existing are so low as to be negligible.
One of the more common objections is the "free will defense", ably expressed by C. S. Lewis in "Mere Christianity":
God created things which had free will. That means creatures which can go either wrong or right. Some people think they can imagine a creature which was free but had no possibility of going wrong; I cannot. [...] Of course God knew what would happen if they used their freedom the wrong way: apparently He thought it worth the risk.
Perhaps we feel inclined to disagree with Him. But there is a difficulty in disagreeing with God. He is the source from which all reasoning power comes: you could not be right and He wrong any more than a stream can rise higher than its own source. When you are arguing against Him you are arguing against the very power that makes you able to argue at all: it is like cutting off the branch you are sitting on.
This is essentially an argument that (C2) does not follow from (A2) and (A5), that although a universe that perfectly meets God's standards could exist, it's not God's fault that this universe doesn't.
But if God is not free, then God is clearly not deserving of worship. It may be deserving of respect, just like any powerful machine, but not worship. Besides, God is usually conceived of as being the perfect example and standard for humans to aspire to (WWJD?, etc.). Apparently we should work to abandon our free will, since that's unGodly... (Of course, that might actually be the case - if there's no evil in heaven, and free will isn't compatible with such a state, then it follows that there must be no free will in heaven. Q.E.D.)
Of course, if God does, in fact, possess free will, (as essentially all theists in fact insist) then apparently Lewis could imagine such a being after all, or at least accepted that one exists. And if it's possible for one such being to exist, then why not more? Why not a whole universe of them?
One possible response is that Lewis specifically meant the word "creature" (that is, something created) rather than the more generic term "being". That is, Lewis would answer the above with, "Of course God both has free will and is without the possibility of doing evil, but God could not create something with this same trait."
This obviously begs the question, "Why not?" The usual class of answers alleges that the problem is really a category error, that there is something special about God's relationship to 'good' that cannot be shared by other beings. This leads directly to the Euthyphro Problem. The question that needs answering is, "Does the Good conform to God, or does God conform to the Good?"
If God can define 'good' and 'evil' however It likes, then of course there's no problem with God always being 'good' - 'good' is whatever God does by definition. Ordering people to kill babies isn't immoral if God does it (1 Samuel 15:3, Joshua 10:40). But now we simply have the ultimate case of "might makes right". There's no real difference between "Speed Limit 55" and "Thou shalt not kill" except that presumably God enforces Its rules better. In the end, the people who collaborated with the Nazis had the right idea, they just picked the wrong bully to submit to.
This isn't terribly satisfying to me and many others, though apparently some monotheists aren't bothered by it. So far as I can see, in this case the only difference between a 'good' action and an 'evil' one is God's arbitrary whim. Even if you assume that God can't change Its mind now, there's no reason why It couldn't have decided that torturing children was the greatest 'good'. God just didn't happen to have chosen that way.
If one asserts that something besides God's arbitrary whims guided the decision that torturing children is 'evil', then one has to ask, "What might that something be, that even God cannot change?" If some things just are 'good' and 'evil', regardless of God's assent, then 'good' and 'evil' exist apart from God, and are recognized, not created, by God. God conforms to 'good', not vice-versa. But in this case, why couldn't created beings similarly conform?
Some theists assert that there is a third possibility, that "God" and "Good" are identical in some deep mysterious way that is beyond human comprehension. This seems to me to be a patent case of special pleading, and sounds a lot like, "Shut up, you're wrong, even if I don't know quite why!" Speaking of which...
The example itself is not even technically accurate. If we use a stream to turn a water wheel, which drives a pump, which raises some water, we can raise part of a stream higher than its source. The energy from a larger amount of falling water is used to raise a smaller amount of water.
(Note: I am not claiming to be smarter than God. I am pointing out Lewis's mistake; creations frequently surpass their creators in specific capacities and for specific purposes. Lewis asserts a logical contradiction where none exists.)
A fourth option is proposed by some theists, that beings simply cannot be created as perfect ex nihilo, but rather must mature into perfect beings. I haven't seen a clear explanation of why this must be. Humans are not born knowing how to walk, or even crawl; they must learn to do so. Most other skills (besides crying and suckling and voiding) need to be learned. But God allegedly created at least two beings (Adam and Eve) that were able to talk, and speak, and feed themselves from the get-go. I haven't seen a clear explanation of why these traits can legitimately be directly implanted while others cannot.
Even if one regards the Adam and Eve story as allegorical rather than literal, the objection stands. Humans are born almost totally helpless, but other animals, even other primates, are born with significant skills. Horse foals can walk within a few minutes of birth. Dolphin calves can swim within a similar length of time. Swimming and walking are, as any baby human can attest, complex operations to master. It's obvious that both animals have inbuilt, instinctive predispositions for these skills. It would appear that any 'learning' that is done is more 'calibration' than actual acquisition of skills from scratch.
Nor are these animals limited to only the skills they are born with. The ability of trainers to teach canters and 'tricks' not seen in nature shows that significant flexibility can be available even if the 'basics' are instinctive. I don't see why the emotional and intellectual skills necessary for at least basic moral behavior could not be 'inborn' in humans...
(Oh, and BTW, what does this 'development' proposal say about, for example, babies who die unbaptized, or miscarry? Are there options for further development in the afterlife? If so, do some 'make it' and others eventually get damned? If not, why not? Are the luckiest people really those who are never born?)
When a fetus is gestating in the womb, its blood supply travels back and forth down the umbilical cord, in and out of the placenta. In the placenta, blood vessels from the mother and the child intermingle and touch. Note that they are not actually connected - they're just very close and tangled together. Think of two intermingled balls of yarn - the strings don't ever actually splice together.
This close intermingling allows oxygen and nutrients to pass into the child's blood, and CO2 and wastes to travel out to the mother's blood to be disposed of. Unfortunately, sometimes other things cross this barrier, like disease-causing viruses. German Measles and HIV are two examples of viral agents that are able to cross this barrier, with devastating consequences.
Why would God allow HIV to cross into the fetus? It has, by definition, done nothing that could possibly warrant such a death sentence. Blocking this transfer would give the child a vastly improved chance at a normal lifespan. Even if HIV is some kind of divine retribution, babies in the womb cannot be guilty of whatever offense brought it upon the world.
I've heard the idea that God cursed the world after Adam and Eve sinned, and now it raineth upon the just as well as the unjust, etc. etc. That never held much water with me. I'm not impressed with a God that either gets so angry that It can't aim straight, or doesn't feel like tailoring It's punishments to only those guilty of offense.
If God's in charge of the universe, then It's responsible for what happens in it, at least the non-sentient parts thereof. A volcano doesn't have free will to bruise; if a lava flow runs over an orphanage and horribly burns a bunch of children, then it's God's fault, pure and simple.
(Of course, some might argue that it's Satan who really does this sort of thing; that it's Satan's free will that God's respecting. That doesn't add up, either; see below.)
Imagine you see someone throwing a baby over Niagra Falls. Would you jump into the water and try to save it? One might be able to argue that one is not obligated to risk near-certain death in a cause with so little chance of success. What if you saw someone throw a baby into a rapid, icy cold river? What if they just tossed the infant into a wading pool?
Whether or not you choose to risk your life to save the child has no bearing on the free will of the person who put the child in danger, though, right? It's at least understandable that you might not risk your life, but saving the baby doesn't remove the free will of the person who put it in danger. That's completely irrelevant.
God has absolutely nothing to risk by saving any of the thousands of children who die every day. God has total knowledge of every one of their situations, and has ample power to save them, at no risk to Itself. Saving them clearly would not interfere with the free will of the abusers in the slightest.
So why is it okay for God to ignore them? To allow children to be abused physically, sexually, and emotionally? We certainly are not allowed to ignore an obvious case of child abuse; what makes it hunky-dory for God to do so? Whatever justification you come up with, I think it's pretty clear that "respecting the free will of the abusers" is a hopelessly inadequate one.
Either that, or we shouldn't try to stop people from hurting others, out of respect for their free will...
Let us assume that there is a God, and It does not want to interfere with the free will of humans (and, perhaps, demons). There are still many ways for a God to ameliorate human suffering without abridging free will. For example, God could work to minimize the ill consequences of an evil choice while still allowing the evil choice to be made, like you saving the baby drowning in the example above.
Imagine a world where everyone is as fortunate as Inspector Clouseau of the Pink Panther movies. Even when the villains try to kill him, some wildly improbable bit of luck occurs to save him. Assassins are knocked onto a serving cart and go rolling out a window, bombs miraculously blow him into a river rather than into pieces, etc.
Imagine if the occupants of a car hit by drunk driver always, miraculously, survived. It doesn't negate the free will of the drunk driver in the slightest, any more than the villains in a Pink Panther movie lack free will just because they are unable to fulfill their desire to kill Clouseau. Failing in what you choose to attempt doesn't indicate a lack of free will. Just because some things are impossible for you doesn't mean you lack free will. Or are you an automaton because you cannot choose to levitate?
Let's assume that an assassin tries to kill someone, and by sheer good luck the target survives. Does that mean the assassin didn't commit any sin by trying to kill the victim? Isn't the choice to do evil the real sin, irrespective of (or at least in addition to) the consequences of the act itself?
We can look at things a different way. If you were walking down the street, and happened to look into a window and saw a child being abused by their parents, you would, of course, be obligated to do something about it. Personally, I think I'd be obligated to do whatever was necessary, up to and including risking my life, to help the kid out, but ask yourself, WWJD? (Would Jesus say, "I don't want to step on the free will of that abuser. I'll just wait, and later on, make sure that kid gets therapy."?)
Now, of course, God is alleged to be omniscient, to see everything that everyone does. If this is the case, God must see many cases of child abuse every second. If you or I were in that situation, we'd be morally obligated to interfere, no? And our interference would not affect the free will of the abuser in the slightest, right?
Now, the point here is not that such a 'Clouseau Universe' would be optimal or even on balance desirable. The point is that such a world would have just as much free will as the one we're in today. The 'you must have bad consequences to have free will' argument doesn't hold up.
There are cases where one could argue that God needn't intervene - if someone's stupid enough to take dangerous drugs, when they should know better, perhaps. I might allow my children to break some of their toys to learn how to take proper care of things. But I won't knowingly allow one child to break another child's toys. I can see where God might permit a drunk driver to harm themselves, but in light of the above, why should God permit a drunk driver to kill innocent families?
The 'Clouseau Universe' is interesting from another perspective in the 'problem of evil'. I'll quote a case made by Richard Swinburne:
I have almost always found in discussion of these matters that my opponents are usually happy to grant me, when I bring the suggestion to their attention, that the states which I describe as "goods" cannot be had without the corresponding evils, and quite often happy to grant that the former states are indeed good states and even that a world is not on balance worse for containing a few of these goods in the mildest of forms with the corresponding evils than it would otherwise be...
The sort of world where so many such evils are removed and which in effect my opponents think that God's goodness requires him to make, turns out - as regards the kinds of good to which I have drawn attention - to be a toy world. Things matter in the kinds of respect which I mention, but they don't matter very much. I cannot see that God would he less than perfectly good if he gave us a world where things matter a lot more than that...
The sufferings of the Jewish victims of the Nazi concentration camps were the result of a web of choices that stretched back over centuries and continents and caused or made possible a whole web of actions and reactions that will stretch forward over centuries and continents (and the same goes to a lesser extent for the suffering of the fawn). Such sufferings made heroic choices possible for people normally too timid to make them (e.g., to harbor the prospective victims) and for people normally too hardhearted (as a result of previous bad choices) to make them (e.g., for a concentration camp guard not to obey orders). And they make possible reactions of courage (e.g., by the victims), of compassion, sympathy, penitence, forgiveness, reform, avoidance of repetition, etc., stretching down time and space. In saying this, I am not of course saying that those Nazi officials who sent Jews to the concentration camps were justified in doing so. For they had no right whatever to do that to others. But I am saying that God, who has rights over us that we do not have over others, is not less than perfectly good if he allowed the Jews for a short period to be subjected to these terrible evils through the evil free choice of others - in virtue of the hard heroic value of their lives of suffering.
But one of the key problems with this is that, by the standard assumptions of most versions of monotheism, we do live in such a 'toy world'. No matter how much evil we do, we can't really hurt people permanently. God will make it all better later in heaven. It's alleged to matter in the sense that we will be punished for the harm we've done to others, but in the sense that Swinburne wants to appeal to, even the good we do doesn't matter. Sure, you can save Jews from the concentration camp and thereby demonstrate compassion and courage. But if you didn't, they'd still be all right... eventually.
It's arguable whether such a universe really has meaning - would really matter - in the way that theists who advance this line of argument say they want. Apparently this universe is, really, a playground where the supervisor doesn't interfere with bullying as it happens. It merely watches with hidden cameras, and just takes careful notes about who pushed who. Once recess is over, it hands out demerits to the pushers and lollipops to the pushees.
There's another, more fundamental issue with this idea. Courage and fortitude are - absolutely - virtues in our world. No argument there. This world is better for having them, and would be a really terrible place without them. But that's this world. In heaven, there won't be any concentration camps, right? So the trait of 'being willing to risk yourself to save people from concentration camps' would not be of any use, would make zero difference. Is courage still a good if there is no call for it? For people in heaven, courage will be superfluous for nothing will be at risk. Indeed, so far as I can see, every single 'good' highlighted by Swinburne - "courage... compassion, sympathy, penitence, forgiveness, reform, avoidance of repetition" - will be, according to every conception of heaven I've ever encountered, utterly unnecessary.
Why are traits which are so completely unnecessary to life in heaven supposed to be the hallmarks of those going there? How important are these 'goods' - how much of a justification for evil do they provide - if they are ultimately irrelevant to life in eternity? (Or, more disturbingly, what does it say about what life in heaven is like if those traits are vital? Or is life in heaven ultimately meaningless, in the exact same way it's claimed a paradise on Earth would be?)
As an alternative, think about what it means if this life really is all we have. Then our choices matter a great deal indeed. Consider the words of Hannah Jarvis, a character in the play Arcadia by Tom Stoppard, as she makes a related point about the pursuit of knowledge:
It's wanting to know that makes us matter. Otherwise we're going out the way we came in. That's why you can't believe in the afterlife, Valentine. Believe in the after, by all means, but not the life. Believe in God, the soul, the spirit, the infinite, believe in angels if you like, but not in the great celestial get-together for an exchange of views. If the answers are in the back of the book I can wait, but what a drag. Better to struggle on knowing that failure is final.
Often people loudly thank God for 'miraculous' cures, or escaping from certain death, or whatever. Frequently God's thanked for sports victories. If an abandoned baby survives being left outside in the cold, then people call them a 'miracle baby'.
So, God gets credit for interceding and, e.g., keeping a baby from freezing to death. But somehow God escapes blame for not interceding to save other infants that die of exposure. It's allegedly within It's power to do so, and no one can claim that God would be unaware of their plight. It must deliberately choose not to save them, no?
If God gets credit for miraculous cures and so forth, then how can It escape blame when such are withheld? To return to the example of someone tossing a baby into a river, people would rightly applaud anyone who dived in to save the infant. Would you not also be furious at, say, an Olympic-class swimmer who saw a child thrown into a river and decided to go off and get lunch instead of saving them? How is God any different?
Some argue that God is in charge of the universe, as creator thereof, and gets to do whatever It likes. We have no standing to challenge any of God's decisions in these matters. God owns all the marbles, and if It chooses to break a few of them, oh well. There are at least two problems with this analogy.
Non-sentient things can be owned, of course. But sentient beings are not, and cannot be, property. I can speak of 'my' wife, and 'my' kids, but I don't own them. I certainly love them, and I have responsibilities toward them (and they to me), but they are not my property. If I take my rake and set fire to it in a fit of pique, well, I may be stupid but I'm entitled to do what I want with my own property. I hope you would object if I tried to set 'my' children aflame, however.
I took a very large part in the creation of my children (though obviously my wife did more of the work). That doesn't give me carte blanche to do with them as I like, though - indeed, it actually places significant obligations and responsibilities on me. I have to raise them, care for them, teach them, comfort them, discipline them, until they can care for themselves. I mostly enjoy these jobs because I love my children, but even if I didn't I'd still owe them my support.
Even if someone didn't voluntarily undertake the work of creating a child (e.g. a father who used a broken condom, or a mother who was raped) they still are obligated to support the child. At bare minimum, they must find someone else willing to care for them. (We find it understandable if a woman gives a child born of rape up for adoption, but even in that case abandoning a child in a dumpster is not acceptable.) And if the caretakers they select for the child prove to be unsuitable, most people would agree they have a responsibility to intervene. (Indeed, above I stated that anyone has a responsibility to intervene when they become aware of child abuse.)
Even the authority of disciplining children is not unlimited. I have spanked my older sons when they did something both (a) unsafe, and (b) that I have told them not to do. But I can't just spank my children every hour because I feel like it, and I can't morally hand out disproportionate punishments. Our youngest son is, at the moment, only a few months old and quite incapable of doing something worthy of punishment.
God allegedly chose to create sentient beings. That doesn't give It unlimited rights to do whatever It wants with them. It gives It substantial duties and responsibilities toward those beings. These responsibilities include passive duties (e.g. not dropping lava on orphanages) and active ones (e.g. preventing child abuse). I do not see a lot of evidence that any Gods are out there fulfilling these duties in any significant way.
On the other hand, if some things just are good, and God is supposed to perfectly conform to these things, then there's no conceivable way God could escape the traditional responsibilities that parents have. If God had created a universe with no sentient beings in it, then It would have the right to do whatever It wanted with that universe. But allegedly God created a universe with people in it, and that would mean God owes us Its care.
So 'authority' isn't a useful or convincing justification for God getting away with ignoring abandoned children. What other justifications might there be?
One 'justification' that frequently comes up is the "Greater Good" or "God Works In Mysterious Ways" argument.
This basically says that God allows bad things to happen because they ultimately lead to greater good in some other place and/or at some later time. Perhaps if God had saved that baby, the doctor who would have unsucessfully tried to save them would not have been motivated to develop a better treatment for hypothermia, or something.
Of course, this proposition is a safe one for a theistic apologist, because it is absolutely, 100% unfalsifiable. There is no conceivable way to prove or disprove it. Even if the results of some occurence are unrelievedly awful, now and for the foreseeable future, an apologist can always argue that the true goodness of the action will happen a long time from now, or is just something we are not smart enough to perceive.
But there's a serious problem with this kind of 'defense'. Essentially, the claim is that God is beyond comprehension, fundamentally beyond human understanding. But that boils down to 'we can't be sure of anything where God's concerned'. I mean, if, by definition, God's incomprehensible, then you can't be at all sure that God's really on your side.
People often try to draw the comparison between children and parents, and us and God. Children don't always understand why they aren't allowed to do certain things, or can't have things they want. Kids have to trust that their parents are looking out for their welfare. Sadly, that's not always the case. Abusive, malicious parents do exist, as I've pointed out. Assuming a God exists, how do we know It isn't more like that than the kind, loving parents I wish every kid had? The rationalizations I've seen for the cruelty in the Old Testament strike me as sadly reminiscent of the rationalizations that victims of child abuse often engage in. "I'm bad so Daddy had to burn me..."
And perhaps 'child and parent' isn't really that good an analogy. After all, optimally kids grow up to be full adults on an equal standing with their parents right? And that isn't supposed to ever happen with us and God, right? We'd be much more akin to pets than children, really. Or maybe not even pets. What if God really is like a shepherd - down to the shearing and slaughtering, too? God's allegedly beyond our comprehension, remember. How would we ever be able to tell if a God wanted to fool us?
Along those lines, it's also worth noting that people have a lot of tendencies that work towards making people think that "things work out for the best" even when that's not so.
So, basically, with this notion there's no possible way to tell the difference between a universe where God intervenes only when it's 'safe', a God that allows good only when it ultimately leads to a greater evil, and a universe with no God at all. All three explain the data we see equally well. Except one is a lot simpler...
Carl Sagan wrote an interesting and insightful essay on this sort of thinking.
In researching the elements of this treatise, I found one that is, in some ways, a nearly parallel essay to this one. But it argues the opposite conclusion, that God exists (or at least, is not ruled out). You can read it here.
Read what good ol' Bertrand Russell had to say. (I don't agree with everything he says there - people can and have been plenty silly and destructive without religion, too - but much of it is spot-on.)