Suppose we came to Paul without any expectations. Suppose we simply tried to put together a picture of earliest Christianity based on our earliest source, i.e., not reading anything in Paul through the lens of later writings.
If all we had to go on were Paul’s writings, I don’t think that it would even occur to us that Christianity was predicated in any way on the teachings of a historical person named Jesus who lived in first century Palestine and who had disciples who passed those teachings along to others. Rather, we would think that Christianity was predicated on the teachings of a small group of men, the primary one of which was Paul, who claimed to have had an encounter with a heavenly being and claimed to have received divine revelations from or concerning that heavenly being. We might not think it all that different from the founding of Mormonism, i.e.,Joseph Smith claimed to encountered the Angel Moroni and received revelations. Like Smith’s Moroni, Paul’s Christ had once been a man who walked the earth, but neither Paul nor his contemporaries had known him personally and what they knew about him when he was a man was only known by revelation.
It is true that Paul talks about knowing other apostles, however, Paul never describes them in terms that would lead us to believe that they had had been disciples of an itinerant apocalyptic preacher named Jesus who had recently tramped about Galilee teaching and healing. If we only had Paul to go on, I think we would assume that these were men who were believers because, like Paul, they had experienced appearances of the risen exalted Christ. Without something else to indicate that Paul knew when and where the man Jesus lived or died or what he said and did during his life, I don’t think we would interpret the reference to James as “the Lord’s brother” as indicating a biological relationship.
Of course Paul is not our only source and it is entirely reasonable to try to make sense of what he wrote in light to what others wrote. Nevertheless, Paul is our earliest source and in many ways our best source so I think it makes sense to think about the picture we get when he is allowed to stand alone. Moreover, many of the early sources other than the gospels seem to paint the same picture that Paul does.
The question that causes me to remain agnostic about the existence of the historical Jesus is this: Do we accept as historical any other person where the earliest and best source seems to point so exclusively to a mythical or legendary figure?