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.
The creation is singing and dancing the praises of its Maker so clearly and so loudly, that Paul says everyone is “without excuse” when they ignore what is “clearly perceived” (Romans 1:20). The fine-tuning of this massive, marvelous, intricate universe we live in is just one of many ways that the creation is declaring the existence and glory of its Creator. Yet the effort to suppress the truth (Romans 1:18) continues in every generation, in a variety of ways.
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.