This seems like an intentional category to exclude. A small movement in Judea would hardly elicit the historians of the day to write books about. But who would write about such a small movement? How bout the ones involved, though even then they seemed somewhat reluctant. Historical writing did not carry the same weight as today and I've found many critics anachronistically will force it on the day. And the apostles seemed most interested in transference of knowledge by the "living voice" and the living Spirit. So how do they get rid of Paul? With a great brush they make his Jesus a cosmic one. Well, they are somewhat right. Though he layed his cosmicness aside and put on his peasant "terra" clothes for a short time. The only way to get rid of the sources we have and their credibility is to exclude them as incredulous by an incredulous means.