Hi Steve,
You're making the assumption that the gospel writers were intending to start a new religion. We don't know that. We know that Paul's ideas were the catalyst for a new religion under the stewardship of the Roman Catholic Church which appropriated them, as they did the gospels. As I said in the article, one possible line of thinking is that they were trying to hold the line against Paul and his new teaching, by partially revealing what had previously been kept secret, in Palestine at least. They would therefore merely have been restating their previously held views, not starting a new religion.
Regarding the Perennial Philosophy, more to come.
As far as I understand it, Islam does not perceive itself to be a new religion. It believes that, because there had been previous errors, a final revelation was needed to Mohammed, in order to correct these, and to return to the original true religion. The biggest error was that Christianity believed that Jesus was God incarnate, and also the doctrine of the Trinity. I didn't know this before, but Salibi says, as included in the article, that the original Judaism split into two factions, Ezra's and Issa's Nazarene Way. Thus I assume that Ezra's Judaism was also perceived by Islam to be an error, in that it was a deviation from the true religion, which was presumably the original Mosaic Torah.
Regarding the historical Jesus, not sure I believe. I'm up to a point trying to learn, and develop my thinking, as I go. My main issue at the moment is that the mythicists jump too hastily to their conclusions, without taking all the relevant information into account. For example, as quoted, Kuhn says that there is not one iota of history in the whole Bible, when, as Salibi demonstrates, all the archaeologists are looking in the wrong place. That's why I appear to be leaning towards the historical side at the moment, to strike a balance.