Unlike any other of the religions of the world, Christianity is uniquely and entirely dependent upon the historical veracity of its holy book, the Bible. The very narratives and prophecies which make up the majority of its pages, rather than being simply a complement to, are the vitals of what Christianity contends as a religion. If the historicity of Adam and Eve, Moses, Abraham, Jesus, and others are not true, then there is nothing on which to found the rest of the Christian teachings.
Paul contends in Romans 1:18-22 that, although God is invisible, his works are a visible proof of his actions and existence. Everyone realizes, in their heart, that there is a God; but many try to suppress this knowledge even from themselves.