Excellent question.
But better yet, when was it ever Jewish?
The original Jesus movement in Galilee is almost a mystery. Apocalyptic traveling Jewish teacher within a community of Zealots doing baptism, does not in any way reflect todays Christianity or that of any of the authors.
To answer your question I will say the day Jesus dies the movement was no longer Jewish, it was over.
What started afterwards, has nothing to do with his movement, and everything to do with the Hellenist who found importance with the mythology surrounding his martyrdom after death.
Not entirely true. There were pockets of actual Jews practicing what he taught, at least until persecuted out of existence by the fourth century or so.
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