Why Genesis as allegory doesn't work

When dismissing Christianity on the basis of evolution, I do this because the origin myth of the Bible is central to the key message of Christ’s role in redemption. If there was no original sin in the Garden of Eden then human beings are not all born with an innate sin nature and there is no need for a saviour. When interpreting Genesis as allegory, the Bible falls apart, because if the original sin is viewed allegorically, then Christ died for a sin that never occurred, to restore man to a state of grace that he never fell from. His death would be in vain. The allegorical view of Genesis is inconsistent with the New Testament. However if viewing Genesis literally, we see that all available evidence contradicts the Biblical account of creation.