I'll take a shot at this.
Richard Swinburne, a Christian philosopher, defines God as follows.
"I take the proposition 'God exists' (and the equivalent propositon 'There is a God') to be logically equivalent to 'there exists necessarily a person without a body who necessarily is eternal, perfectly free, omnipotent, omniscient, perfectly good, and the creator of all things.'" (The Existence of God, p. 7)
An atheist is a person who believes that God, so defined, does not exist. I will present four quick reasons for being an atheist.
Reason 1: Free will does not exist.
If God exists, then God has free will, per Swinburne's definition. But it is extremely difficult to say what free will is supposed to be.
Obviously not too difficult, if you see it in Swinburne's definition, where it's not explicitly stated.
Indeed, the notion of free will appears to be positively incoherent: An action must be either caused or uncaused, but it is not free if it is caused or if it is not caused. Therefore, neither free will nor God exists.
"Free will" (in this case apparently defined as
an action that is free) is only incoherent because you make it so, by defining an action as something necessarily not free. It is only by your defining it that way that there is no action that can possibly be free. Are people not given to act freely?
Reason 2: Consciousness requires a brain.
If God exists, God is conscious without a body, per Swinburne's definition. But everything conscious that we know of has a brain. It is more reasonable to conclude that consciousness is a process that occurs in brains than to believe that it is a substance in itself which can have an existence independently of the brain. Therefore, God does not exist.
Again, you've defined only those things that have a brain as being conscious, but we have observed conscious behavior in plants, insects, machines, and potentially, if not actually, in computers, all without brains. It's also the case that, while we conceive we need brains to conceive, that
is us conceiving.
Reason 3: The problem of evil disproves God's existence.
If God exists, he is omnipotent, omniscient, and perfectly moral, per Swinburne's definition. If such a being existed, then evil would not exist. But evil does exist, in overwhelming quantity. Some apologists attempt to get out of this argument by deploying a free will defense, but as we have seen, free will does not exist. Therefore, God does not exist.
Being limited to Swinburne's definition, and taking any one definition of "evil" as useful, this is the logical conclusion to the Problem of Evil outlined. I'd only mention that, as pointed out earlier, it's a
very particular image of God, and that alone makes it of questionable worth for being a reason for atheism, at least my atheism. If I can look at one image of god and know it to be substantially different than that held by a billion other people, why would this one argument be a
good enough reason to call myself "atheist"?
Reason 4: Occam's Razor "shaves off" the claim that God exists.
Given the lack of objective positive evidence for the existence of God, it is simpler to attribute belief in God to the intellectual vices and emotional needs of the believer than to an actually extant deity. In cases like this, we can safely appeal to Occam's Razor to "shave off" the claim that God exists, just like we do the claim that leprechauns exist. Therefore, in the same sense in which we say leprechauns do not exist, we can reasonably say God does not exist.
When you delve into the philosophy that supports an epistemic view of the world, though, Occam's Razor can cut both ways. Between cause and effect being reduced to inference, and the uncertainty of relying on authorities, those authorities, such as "truth," and "objective reality," are things the razor takes aim at. When it's truth and reality of the world that gets summarily "shaved off," what you're left with--all that remains--is... the world, true and real. That's where people begin to find god.
I look forward to your responses.