There are many reasons to be an atheist.
There are arguments from materiality, from nature, from the philosophy of logical positivism, and lots of little contradictions in religious scripture which you can have endless pedantic fun pointing out to believers.
But when you get down to it, these arguments are just artifacts of non-faith, in exactly the same way that the rejection of such arguments is an artifact of faith.
I didn't become an atheist because of any of these arguments. And I wouldn't expect anyone to reject their beliefs on the basis of these arguments.
So why atheism? It's actually very simple. The most obvious reason to accept atheism is the simplest.
And it's simply this: every religion is clearly and evidently the invention of humans.
There is nothing in any religion that's even mildly convincing as an argument for the existence of a deity.
Every piece of scripture that is oh-so imperfectly worded, every purported miracle that turns out to be a hoax, every claim of Godly omnipotence that can't seem to do anything more than bestow a "good feeling" in its believers . . .
This is supposed to be an all-powerful, all-knowing, omnipresent God. So why is he completely and utterly invisible, literally and metaphorically?
Why are so many of the claims of His believers so patently false? Why can't they agree on even the basics of the "truth" that has apparently been divinely revealed? Why are there are no miracles? Why are there no healings?
But most importantly, why is there no power in religion? It all so embarrassingly weak.
If all this theology is the construct of a God, then that God is hardly worthy of our praise.
And if it isn't . . . well that would certainly explain a lot, wouldn't it?