Who were the "Petrines"? And why supercessionism?

[rakovsky]

I believe then the question is not of the grafting of Judaizing ideas onto an originally non-Jewish phenomenon but rather the reforms of a group that is accommodating to pagan-background interest (Marcion and/or his predecessors) leading to a conservative whiplash that eventually causes the doctrines we know as supercessionism to be articulated.

The NT is full of references to the Tanakh, so I agree that it was an originally Jewish phenomenon.

I think you are saying that Paul's ideas were "the reforms of a group that is accommodating to pagan-background interest (Marcion and/or his predecessors) ". A big difference between Paul of 45 Ad and Marcion of 120 AD was that for Paul the Tanakh and Torah were crucial bases for Christianity as he talked about in Romans and Galatians. His main explanation of his supersessionist idea is in Gal 3 and Rom 9 to 11, and even there he justifies his beliefs by connecting the Gentiles to Abraham as their father and saying the Gentiles were grafted in. So he was making accommodation by saying gentiles can stay gentiles and don't need to follow the food rules or get circumcised. Marcion on the other hand was not actually a supersessionist, since he denied that the torah was ever legitimate in the first place, so it couldn't actually be either superseded or replaced.

Still, I think some basic aspects of Paul's ideas can be found in the gospels and Acts, like where James decides Gentiles don't need circumcision and the gentile soldiers announce Jesus as son of God at the passion.

[rakovsky]

Peter, check out 1 Peter 2:

"Once you had no identity as a people; now you are God's people. Once you received no mercy; now you have received God's mercy."

I think Peter is talking about Christians here, calling them God's people, a name previously used just for the Jews.
I agree 1 Peter is one of the earlier writings. I think that the supersessionist ideas were already there.

Arguably, supersessionist is already existing on the Tanakh when it makes predictions about the Gentiles as having a new, more holy status like in the end of Isaiah 53.
One Jewish writer explained it to me like that even in a nonchristian apocalyptic context. Early Christians had these kinds of apocalyptic ideas.