The Jerusalem Church developed into the Ebionites and continued to wait for Jesus to return in "their lifetime" with the resurrection of all the saints. They did not believe in the virgin birth and saw Jesus as a mortal. The Ebionite movement was wiped out pretty much in the second revolt under Simon ben Kokba. Neo-platonic thought moved another aspect of Jewish thought with the help of the Johannine literature toward Gnosticism and Mystery Cultism( Paul). Logos Christology became predominate in the second century while Paul's writings became old hat. Marcion rejected the Jewish background of god and created the first Christian bible. Other groups of churches reacted against Marcion and Paul's writings came back into popularity as well as the pseudepigraphical ones attributed to Paul and Peter/Jude and John. Various forms of Christianity competed with each other creating more and more writings attributed to the Apostles. The gospels were attributed to Matthew , Mark, Luke and John. Jesus became more and more divine and everybody got together and created a bunch of trinity theories. The politicians won.
Historical researchers in this field generally see Jesus as a wondering Cynic teacher spouting out words of wisdom and folk philosophy. It does seem to be the core of what developed into Christianity and the hypothetical document Q. Jesus was a human who came from Galilee. Galilee was the only area in Palestine that was forcefully converted to Judaism. Even so only 50% of the population was Jewish. Rabbi Hannina ben Dosa and Honi the Circle Drawer were very similar characters to Jesus. Simon ben Dosa called god Abba like Jesus ( meaning Daddy instead of a reverential “father”). He spoke openly with women as Jesus did.
The reason that historians posit these as historical is because they are contrary to the believing Jewish or Christian movement. You can see evidence that the church tried to cover up the fact that Jesus was baptized as others to get rid of sin. Historical Jesus research is a discipline to explain the sociological development of what became Christianity ... what historical kernel was the catalyst for all the mythic construction. It is commonly understood that the bible is mythic in seminaries and theological schools like Princeton, Yale, Emory, Vanderbilt. The bridge from Theological school and seminary to the church and its members is “teach it as truth and avoid the lack of factual basis.” So statements are demythologized and taken into a philosophical meaning rather than a grounded factual historical meaning. Virgin birth does not really mean a women had a child and was a virgin. It becomes a story to honor Jesus as both god and man. So you have pure historical work. Then the theologians that try to make it still meaningful and then the preachers to present it as literal. When I was in seminary my mentor (Hendrikus Boers) who wrote, “Who Was Jesus?” was a Marxist atheist from South Africa. He would point to people like Jurgen Moltmann (theologian) as a fraud that needed to be exposed. Then there is the whole moderate movement that tries to salvage some Christianity out of the historical/critical conclusions. Crossan was on the Jesus Seminar team. He knows Jesus was simply a person who got into trouble and was removed from being an irritation. The people who cared about where Jesus was buried did not know where he was buried. The people who did know where he was buried (communal grave) did not care.
Sophisticated Theology affirms the historical conclusions of historical/critical research as well as science but as with the trend since the neo-orthodoxy of Karl Barth and Rudolf Bultmann wish to convert the theological and ethical meaning of the unhistorical scripture into eternal and continued truths for the church. The stories become no more than hyper-fables. And some of their truths are dangerous to our culture.