I agree that there was no established Christianity for the first few centuries of that religion. Instead, there were different sects vying for influence, including a more heavily Jewish version of Christianity, led perhaps by James and Peter in Jerusalem. That sect was wiped out in the Jewish-Roman war, which led to the promotion of the Pauline/Catholic and Gnostic sects (or cults). Catholics, which is to say totalitarian “universalists” appropriated Paul’s quasi-Gnostic, largely pagan theology and welded it to a self-serving, duplicitous interpretation of Judaism, using the canonical gospels to tie them together.
So, while I agree that the gross expropriation doesn’t fall on all the proto-Christian cults operating in the first couple centuries CE, it does fall on the official, Catholic kind that took over with its taming of Paulinism via the canonical gospels. Certainly, the Gnostics and Marcion didn’t pretend that they spoke for Jews. Marcion explicitly disavowed the Hebrew scriptures, recognizing the plain difference between Judaism and the emerging, Hellenistic theologies. The proto-Catholics formed their canon in opposition to Marcion’s anti-Jewish one.
Paul didn’t originate all of Christianity, but he was the model for all, much later obnoxious televangelists. As I show elsewhere (link below), his bizarre arguments paved the way for so many later “universalizing,” casuistic Christian compromises and distortions. Paul was essentially a sophist. Again, I agree that his proselytizing sustained that syncretic form of Christianity, but it’s not the case that without it, Christianity would have “remained a small sect within the framework of Judaism.” As I said, the Jewish-Christian sect was wiped out in the war with Rome, leaving rabbinic Judaism, Gnosticism, and the obnoxiously syncretistic, power-hungry, Pauline/Catholic Christianity.
Just ask yourself this: If Paul is so important to the founding of Christianity, as he clearly is, why doesn’t the NT include any Gnostic or Mithraic texts or references, when Paul was clearly influenced by them? Why not some references to the other dying and rising gods? Why only the pretense that Pauline Christianity follows purely from Judaism (from the OT)? The pattern of the Catholic Church’s co-opting of pagan traditions and of its lack of crediting those influences was established early on, in its absorption of Gnosticism via its inclusion of Paul’s letters themselves. As Elaine Pagels points out, Paul was a proto-Gnostic. But that Gnosticism is just duct-taped to the Hebrew scriptures to form the Christian Bible, as if Jewish monotheists had no objection to this paganism.
Historically, many ancient Jews weren’t so tribal and xenophobic. They were curious about Greco-Roman culture. But when Jews were down and almost out, due to their failed revolts against the Roman Empire, the proto-Catholic Church promoted a bastardized form of Judaism, not to honour Jewish traditions but to supplant them. It’s what we today would call a theft of intellectual property, and indeed it’s a lot like the woke vandalizing of pop cultural heroes in the toxically feminine sequels that Hollywood’s churning out. A thief of intellectual property may keep the original work but pretend that that work no longer belongs to the original artist. That's what's so gross here. It's a sin against artistic ideals.
You say the Church would have needed to repudiate Judaism to justify excluding the Jewish scriptures from the Bible. But this leaves out the option of grossly expropriating the Jewish tradition with a Romanized, Pauline interpretation of it. That’s the option taken by what became official Western Christianity.
