1. Religious reason: There is no proof for God outside of the bible. Once I dropped my belief in the Bible, the Judeo-Christian concept of god fell apart like a house of cards.
For me: 1 a. The God of the Bible is severely flawed. He approved slavery, genocide, rape, slaughter of innocents. He is jealous of nonexistent gods, his tests upon mankind are unjust or confusing (freewill but only if you do what I say), he demands punishment on children for the sins of the parents (Adam and Eve, David), women were property under his laws.