Upvote:1
I had a wonderful discussion about the Jewish point of view of Melchizedek because of Paul's reference to him as an order of priesthood. That discussion and Paul's letter to the Hebrews probably answer your question. From Hebrews 7:11(KJV):
If therefore perfection were by the Levitical priesthood, (for under it the people received the law,) what further need was there that another priest should rise after the order of Melchisedec, and not be called after the order of Aaron?
Paul's letter is thought to be written to Jewish Christians, and its nature would support that. Given this perspective, "Mainstream Christianity" and "Jewish Christianity" were already in the process of separating by AD 63.
Thus, to answer your question, the cause was Jewish Christianity's continued adherence to the Mosiac Law and assumption that the Levitical and Aaronic Priesthoods superceded Christ Messiah.
Upvote:1
The earliest cause of the breakup between Christians and Christians was the observance of Pascha. Some observed it as a floating observance on whatever day of the week it fell. This means the traditional Good Friday to Sunday Resurrection could be Tuesday, Wednesday, Thursday, or any other combination of 3 days and 3 nights (sign of Jonah). Others, led by Rome, beginning with Sixtus I to Anicetus to Victor observed a fixed Sunday sunrise resurrection.
see this article for more explanation, though some details are incorrect: https://en.wikipedia.org/wiki/Quartodecimanism