There's the obvious and much discussed reason; there's no proof God exists. Religious people arugue that there is. That the miracle of life proves the existence of God. Then others claim that everything evolved over years, and the existence of God is just an easy explanation.
Then there's the complex reasons:
If, theroetically, part of a religious book or belief could be proved wrong, then theoretically, the whole book could be wrong!
There's a statement in many religious books which claims; "God is all powerful". This, is easy to disprove. Using paradoxes I can disprove it; If God is all powerful, then how come he can't build a wall that's too high for him to jump?
So, if he can build a wall that he can't jump he isn't all powerful because he cannot do it. But if he can't build a wall that he cannot jump, then THERE's something he cannot do!
Although this may seem small, many of the things we know about religions can be disproved in similar ways.
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