A religious answer to the question, "Why are you an atheist?" can be found in countless places in the Bible (or Quran or Talmud). The various holy books are, if people actually bother to read them, the best manuals for creating atheists that you can find. In my life, I've read the Bible three times in its entirety and every time, it has strengthened my disbelief. Since I know the Bible better than the other Abrahamic holy books, that is where I find a good deal of my inspiration.
So it is with the answer to the question, "Why are you an atheist?" For a great answer, look to 1st Corinthians, chapter 13, verse 11 -- I happen to prefer the King James Version for its language, so here goes.
"When I was a child, I spake as a child, I understood as a child, I thought as a child: but when I became a man, I put away childish things."
That always makes me smile.