My first response would have been “I was born that way,” except that might inspire singing and dancing to a tune that includes far too much religion. So I won’t go there.
(Although it’s true: my parents separately figured out the flaws in religion in their early teens and it was absent from the household I grew up in. I first heard the word “god” in kindergarten and had to go home and ask what it meant.
And yes, I know just how lucky I am to have been spared all that trauma. My parents are amazing.)
But I like to think I’d have been an atheist anyway, just because anything else would clash with my moral code. It’s impossible to value the achievements of our extraordinary species if you believe we were built from some imaginary dude’s blueprint. I can’t feel grateful for the universe I live in if it was slapped together for the sole purpose of giving humans somewhere to stand.
I don’t think I could revel in the crazy-huge range of people we humans include if I thought that some kinds of people were more valuable than others. And I couldn’t be awed by our incredible ability to use science and rationality to explore our existence if a corner of my mind rejected the need for evidence.
Most importantly, I don’t believe I could keep trying to do the right thing, to make the best choices — even when they’re hard ones — if I simply followed a list of proscriptions written by someone else. Ethics are too important to leave to religion.
Therefore, atheism. Anything else would be immoral.