Why one would call Christianity a sect within Judaism?

Answer 1

Originally, Cristianity was started as a sect of Judaism. As time went on they split away, having nothing to do with the Jews. Nowadays, it can't be considered a sect of Judaism, but at one point it was. That was its pull. That's why so many Jews were sucked in.

Answer 2

It began as one, but by the end of the second century, there was what might be described as a divorce. One hypothesis is that the final straw was the Jewish revolt led by Simon Bar Kochba in the second century. Bar Kochba drove the Romans out of Jerusalem, several leading rabbis declared him to be the Messiah, and a call went out to the Jews of the world to support the revolt. Rome countered by cracking down on all Jews with intense persecution as well as raising legions from across the empire to smash the revolt. Christians said, in effect, "wait, we've already got our Messiah, don't blame us, we're not part of this."