"C.S. Lewis wrote some fiction, yes. Some of my favorite stuff is his fiction stuff: The Chronicles of Narnia, Screwtape Letters, etc. - but he wrote plenty of non-fiction: Mere Christianity, The Weight of Glory, etc.
The New Testament is constantly under attack and its reliability and accuracy are often contested by critics. But, if you want to disregard the New Testament, then you must also disregard other ancient writings by Plato, Aristotle, and Homer. The New Testament documents have been proven to be better-preserved and more numerous than any other ancient writings. Because they are so numerous, they can be cross checked for accuracy... and they are very consistent.
For example, there are presently 5,686 Greek manuscripts in existence today for the New Testament alone. If we were to compare the number of New Testament manuscripts to other ancient writings, we find that the New Testament manuscripts far outweigh the others in quantity.
Because of this, the internal consistency of the New Testament documents is about 99.5% textually pure. In addition there are over 19,000 copies in the Syriac, Latin, Coptic, and Aramaic languages. The total supporting New Testament manuscript base is over 24,000.
Almost all biblical scholars agree that the New Testament documents were all written before the close of the First Century. If Jesus was crucified in 30 A.D., then that means that the entire New Testament was completed within 70 years and written primarily by eyewitnesses. This is important because it means there were plenty of people around when the New Testament documents were penned who could have contested the writings. But, we have absolutely no ancient documents contemporary from the First Century that contest the New Testament texts.
If you acknowledge the historicity and writings of those other individuals, then they must also retain the historicity and writings of the New Testament authors...and that makes it more than allegory.