I read somewhere that the origins of Christianity lay in Alexander's conquest, which ended the temple state. That vacuum gave rise to collegias, groups of like minded individuals who gathered regularly to share their interests.
That a person could achieve salvation through the mind without regard to their social origin was the innovation IMO. This was grounded in Greek philosophy, but made more accessible by writers such as the author of Paul. Judaism was unique in that it was literary, which caused many people in the ancient world to conflate Judaism with Greek philosophy. Also it was available for people to study, as opposed to oral traditions. Where there was a Greek-Jewish synthesis going on, eg Alexandria, writers such as Philo explored these issues. The early Christian writers were a continuation of this process.
It helps to understand what the logos is, because that understanding demonstrates the significance of the messiah figure. Reading Doherty again, I see that for all his insight, he does not understand the logos. Understanding is an experience greater than the sum of its parts. Whenever we understand anything, we begin by learning points or facts about something and eventually we sense a wholeness that informs us we're achieving an understanding. That understanding is Transcendence and those steps, items and facts that lead us there is Intelligibility. Or, the One and its Logos, or the Father and the Son. This is Platonic philosophy cast in religious terms.
Now, while all this is going on, there are also sages wandering around exhorting people to lead ethical or spiritual lives. Eg the Cynic sage, a very Jesus like figure.
It seems quite possible that people started seeing the figure of the sage, the Jewish messiah, and the logos as the same thing. Much more likely than the result of the efforts of a single man.