who Jesus Christ supposedly was

He may never have been an actual person or he may have been. That's really unimportant at this point as we can not know and the movement created from his supposed actions have progressed far beyond anything he may have claimed if he ever existed.

and why Christianity started

It started as an anti-temple sect of judaism and ended up triumphant as a result of historical circumstance (including Pauls embrace of non-jews).

Apparently he was a rabbi who pissed off mainstream Jews by declaring that the temple would be destroyed someday?

As one of the anti-temple sects Christianity was well-placed to take advantage of the destruction of the secont temple cult by the Romans.

I am confused if he was supposedly a committed Jew

Early Christian sects were distinctly Jewish, yes. Heavily influenced by neoplatonism though which explains the rest of your question.