First we must keep in mind the difference between scientific evidence and historical evidence.
Scientific evidence is subject to logic, and is repeatable. It forms the basis of the "hypothesize and test" methodology.
Historical evidence is something from the past. It may be nanoseconds or centuries, but by the time you know about it, it's past. As such, it is subject to witness. Until modern times, that witness was largely human. And the most durable witness was the written word (verbal tradition being subject to drift). Human witness is flavored by the individual. Written documents are fragile, and must be copied from the depths of time to be available to us today.
First let us brainstorm some possible sources of evidence for God and Christianity:
- The Bible
- Changed lives
- Religious experiences
In my opinion, number 1 is the most convincing evidence. I read a lot. Mostly science fiction. Science fiction covers a wide array of settings (one of the reasons I like it). You'll find mysteries, romance, and adventure stories. Really good sci-fi will create a subtle and intriguing universe by presupposing one or more changes in society (usually technological). It is then up to the author to apply these changes in a consistent manner. The best books will be fairly successful in this attempt. The best of the best will write several books.
What does this have to do with the Bible?
The Bible also presupposes a change from most people's point of view. The idea that there is a God who created everything, and who has interest in what we do (as opposed to a deist notion of God). The Bible contains history of God's interaction with people, and people succeeding and failing in obeying God. The New Testament is a particularly powerful set of stories detailing the life of the God/man - Jesus Christ. And letters circulated in the early church.
Then consider that the Old Testament was written over the period of hundreds of years, and the New Testament over several decades. The total number of authors is in the dozens. Different cultures and languages. Yet one coherent story.
No single book can compare with the Bible (in terms of units sold, interest, secondary material generated, etc.). The closest thing would be to compare a series of books, ideally a series with multiple authors (since the Bible has many authors). Most series just aren't that good. Tolkien's famous Lord of the Rings trilogy is honored for making fantasy popular. It reads like a linguist creating characters and a plot for people to speak his invented language. Rowling's Harry Potter series has managed to come close to rivaling sales of the Bible. We'll see how popular it is in a generation or two...
Those series have a single author. The only series I am familiar with that have multiple authors are the Star Wars and Star Trek franchises (not counting the romance lines). These series are pretty bad. Fans tolerate them because they are hungry for material. The Star Wars books are particularly bad (except for Zahn), because Lucas cares so little about canon. I remember a particular ordeal where a series of comic books made huge impact on the canon. Zahn then had to try and reconcile some of this silliness into his own books. I'll only mention the Star Trek television programs.
These are teams of people working together to create coherent content in a short period of time. When multiple authors are involved, there is at least some communication; and often a "lead architect" or content gatekeeper.
Yet these people can't create a coherent story.