>> “No. Christianity is definitely a separate religion from Judaism” <<
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No, it certainly is not! (not from real Judaism as described in Torah)
Separate from the traditions of men, yes, but Yehova’s commanded worship has nothing to do with any human tradition, Jewish nor catholic.
>> “. There is no indication in the OT that the prophets who spoke of a Messiah meant someone who would establish churches of a competing religion in the heart of Jewish territory.” <<
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And nobody has done that. You are stupendously misinformed.
Does the Torah describe Judaism as merely believing in the Messiah? Because that is the only way they are the same religion.