Why Don't You Believe in God?

When I am asked 'why don't you believe in God?' my first reaction before giving my reason would be 'I presume you mean the God of the Bible; the God of Abraham, Isaac and Jacob; of Moses and the Prophets; Jesus and the Apostles. The God of Judaeio-Christian tradition; to a lesser extent, the God of Islam.' Once this has been determined I begin 'Charles Darwin's theory of evolution by natural selection has been proven beyond reasonable doubt. The evidence of descent with modification is incontrovertible. Now then, let's assume that we both agree that evolution is a fact, for the sake of argument. What does this mean for the God of the Bible? Well, it would mean that Adam and Eve, from whom all humans are descended according to the Genesis account, never actually existed. We don't see humans coming from two individuals in evolutionary history. Presume that's correct. This would mean that the account of Adam sinning against God in the Garden of Eden did not occur in reality. So the original sin never happened. The Bible teaches that all are born in sin, passed down from Adam to each successive generation. If Adam never existed then this sin nature has not been inherited by all humanity. Come to the question of Jesus, sent to die for the sins of man, specifically as the Apostle Paul puts it, the sin of Adam. The reality of evolution would have Jesus Christ being executed for a sin that never occurred. The entire Christian faith is founded on redemption for the sin of man. But if man has not inherited sin from Adam, who evolution who have us believe wasn't real, then the moral code of the Bible is the product of a falsehood. It has no foundation in reality and therefor cannot be trusted to determine what moral or immoral, nor for that matter has it any right to declare any behavior sinful or wrong. The Bible is held to be an accurate portrayal of the character of the Christian God. But if the very foundation of its moral code (sanctioned by God by the way) is a falsehood, then what credibility can, or should, I lend to anything else it says about God?
To summarize, if evolution is true: Adam didn't exist. There was no original sin, and no sin being passed on to all men. If we are not born in sin we don't need a saviour. Jesus' death was in vain. If we are not born in original sin then the entire redemptive narrative of the Bible seeks to restore man to a state of grace from which he never fell, nor was he created in.