Paul said, "As in Adam all die, even so in Christ shall all be made alive." If the Old Testament is a myth, there was no such person as Adam. As a result, Paul was bearing false witness, and Jesus Christ, who believed the Old Testament, was part of the conspiracy. That means the Christian has no message to share! If Genesis is a myth, then nobody died in Adam, and nobody, as a consequence, will be made alive in Christ.
Learn in this audio why I believe the Bible to be the Word of God and how the Great Author went through great pains to make sure we had a rock-solid foundation upon which to build our faith.