Atheists generally don't believe in God because there's no scientific proof of God's existance. This lack of proof doesn't disprove the existance of God, but neither does the Bible, Koran, Bhavagad-Gita, whatever, prove there is a God.
Faith is just faith, can certainly be a powerful force in life, and the faith may be true. Or not. Faith is believing in something without proof.
Some religious point to the complexity and "design" of the universe as proof there must be some type of divine creator. Maybe they're right, this universe couldn't occur randomly. This argument begs the question, though. If you allow that this universe couldn't just "happen", that it had to come from somewhere, where did God come from? Well, God just is/was/will ever be is the standard answer. If you can accept that, you can accept the non-God beginning/evolution of the universe. It's not hard in logic to go from "God just is" to "the universe just is".
As an aside, there's a recent alternative theory to the Big Bang that hypothesizes an oscillating universe with no beginning or end.