The writers of the gospels were Jewish people who represented the second or third Christian generation. They wrote in Greek, not Aramaic, which was the language Jesus and his disciples spoke. The gospels — at least the first three: Mark, Matthew and Luke — are the products of the Synagogue, which had shaped the Jesus story dramatically over the 40-70 year period that transpired between the crucifixion and the gospel writing tradition.

I disagree with Tom Harpur's thesis, for I do not think Egyptian mythology can shape the Jesus story in as short a period of time as existed. I note that Paul writes in Galatians, a book that is usually dated in the early 50's, that he had conversed with Peter and other "pillars" of the Christian movement within four to nine years of his conversion, which scholars date one to six years after the crucifixion. Mythology needs more time than that to develop.

People need to embrace the fact that the Jesus story was kept alive, recalled and celebrated in the Synagogue, for that is where the followers of Jesus worshiped every Sabbath. The Synagogue and the Christian Church did not separate until 88 C.E.

I am quite sure Jesus of Nazareth was a person of history in whom and through whom Jewish people believed that they had experienced the presence of the holy God. It was in that experience that Christianity was born. The earliest articulation of that faith came from Paul who wrote, "God was in Christ, reconciling the world to God."
How we tell the world of the meaning of that experience is still what Christianity is all about.