Debates about supersessionism frequently flare up in America between evangelicals (generally pro) and mainliners (generally contra). They look and sound like an outgrowth of the early modern law and grace controversies. I would like to argue the real debate is elsewhere.
The following etymology is a helpful frame for what we want to talk about: “The word supersessionism comes from the English verb to supersede, from the Latin verb sedeo, sedere, sedi, sessum, ‘to sit,’ plus super, ‘upon.’ It thus signifies one thing being replaced or supplanted by another.”
Recent scholarship suggest that the definitive break between Christianity and Judaism is later than first supposed, much later, perhaps as late as the sixth century. Things get even more complicated when you consider the following passage from Richard John Neuhaus:
“In fact, the early Christians, both Jewish and gentile, made no secret of the Jewish grounding of their faith. The second century Marcion who pitted Christianity against the history of Israel was condemned as a heretic. Many pagans did deride Christianity as a ‘Jewish sect,’ which did not prevent its continuing growth. Moreover, those Jews who did not accept Jesus were themselves involved in reinventing Judaism after the destruction of the second temple in 70 ad. It is not too much to say that there were two competing versions of the history of Israel that were presented to the world: what became known as rabbinical Judaism on the one hand and the Church on the other.”
This means that both rabbinical Judaism and Christianity in some sense superseded Temple Judaism as rival versions of Judaism. Therefore, you get two supersessionisms for the price of one, or, alternatively, two legitimate theological developments.