- We have reliable copies of the texts
- The authors were in a position to report, accurately
- The writing was early enough
- The writers wanted to record things faithfully/accurately
- Oral traditions were accurate
- The writing fits the cultural patterns of the day
- The material isn't too flattering
- The material doesn't attempt to force closure to issues of the day
- Non-biblical testimonials generally confirm scriptural accounts
- Archaeology generally confirms scriptural accounts
- The early church testimony generally confirms orthodox views of scripture
If I were an orderly sort, I would probably address the cultural patterns evidence today. But I am actually going to address the "material isn't very flattering" argument instead, getting to cultural patters and language in a future post.
I read few modern autobiographies. Perhaps this is something I should address, but I've found that the few I have read lately tend to either aggrandize the subject, or blame failure on others. I find this lessens the credibility of the authors; nobody is perfect (save God Himself) and pretending to be so makes me less likely to buy what the author is selling.
This tendency to elevate oneself in a piece of literature (either biographical or autobiographical) is not new. People for, well, ever have tried to strengthen their case by presenting flattering items more prominently and minimizing or hiding negative items. Ancient legends focus on power and heroic deeds, not on failings. The heroes do the work, make the discoveries, point out the flaws in others. The Gospels, though, are different.
The Gospel writers left in lots of things that were very unflattering, and the NT in general reveals many flaws in the leaders of the early church. For propoganda purposes, this would have been a suidical approach unless, in fact, the events were understood to be accurate.
For instance, James and John got a bit full of themselves. Peter, the man on whom Christ built His church, got a smackdown and later denied Jesus publically. Thomas was a doubter.
The Apostles weren't the first to find the empty tomb, which indicates truth telling as women were not considered credible witnesses at the time.
And the Gospels are not alone in reflecting truth over ego. Inter-church squabbles are aired for all to see in Acts 11 and Acts 15, for instance.
Taken as a whole, the authors come across as honest men trying to relate honest history. They did not take the opportunity to "clean up" the leaders of the early church or make them look special. There is no cover up, or shifting of blame to others. While not proving much of anything, it certainly adds credibility to the notion that these guys were trying to be accurate, not propagandists.