Updated 5 hours after posting to expand Schweitzer quote.
The approach I like to take is one I learned from the way historians (certainly many of them at any rate) investigate other topics, whether in modern, medieval or ancient times.
I have used the example of Alexander the Great before, so for convenience I use it again here. It’s a safe bet to say that the existence and conquering career of Alexander is a “fact of history”. We have primary evidence from his own time still surviving (e.g. coins) and testifying to his place in history. We have much other evidence for major cultural, economic and political changes throughout the Middle East that are most cogently explained as the result of his conquests. So when we read secondary sources about him we have supporting knowledge that assures us that these sources are about someone real. We might call this sort of supporting knowledge “external controls” that we can bring to our reading of the secondary sources.
The problem with studying Christian origins as if Jesus himself were the historical founder of Christianity is that we have no similar controls to support the New Testament narratives. This is why, after discussing the problems with using Josephus and Tacitus as evidence for the historical Jesus, Albert Schweitzer wrote:
In reality, however, these writers [those arguing for the historicity of Jesus against mythicists] are faced with the enormous problem that strictly speaking absolutely nothing can be proved by evidence from the past, but can only be shown to be more or less probable. Moreover, in the case of Jesus, the theoretical reservations are even greater because all the reports about him go back to the one source of tradition, early Christianity itself, and there are no data available in Jewish or Gentile secular history which could be used as controls. Thus the degree of certainty cannot even by raised so high as positive probability. (From page 402 of The Quest of the Historical Jesus, 2001, by Albert Schweitzer.)
New Testament scholars have sometimes been pioneers. The attempt to define criteria of authenticity was in fact an attempt to articulate more precisely and rigorously things that in most other areas of history were determined in much the same way, but with a far greater degree of intuition and instinct. (Dr. James F. McGrath, Clarence L. Goodwin Chair in New Testament Language and Literature at Butler University, Indianapolis, Jesus and the Criteria of Authenticity Among Friends and Enemies . . . )
In the same post, Dr. McGrath explains that the pioneering method of applying more clearly defined criteria of authenticity can be used to hopefully understand the great mystery that started Christianity:
While it is surely true that an attempt to find an uninterpreted Jesus amid the interpretation of the Gospel authors is implausible, it does not follow that criteria of authenticity are useless. What we seek to catch glimpses of are Jesus as he interpreted himself, and Jesus as his disciples interpreted him prior to the changed perspective resulting from Good Friday, and from whatever subsequent experiences and reflections persuaded them that he had been raised from the dead and exalted to God’s right hand.
My earlier post complaining about the absence of known facts about the life of Jesus and the consequent need for historical Jesus scholars to try to find some through criteriology was misguided. It appears that historians who are so backward as to seek explanations for known public facts are “fact fundamentalists” and have much to learn from New Testament pioneers.
. . . . . the issues Allison and others raise are fatal for the historical Jesus enterprise, but are fatal for the misguided and futile quest for certainty that “fact fundamentalists” have brought with them into the discussion. (Jesus and the Criteria of Authenticity . . .)
What is one of the issues raised by Allison according to Dr McGrath that is fatal for “fact fundamentalists”?