What is Supersessionism?

It seems to me that a Catholic cannot help but be a crass supersessionist unless he recognizes the value that keeping the Law of Moses can have for the Jew who would serve G-d as He revealed Himself through His Incarnate Son.

There are basically three ways something can be perfected:

1. By something else that combines with it to bring it to perfection, as, for example, the spices that perfect a meat stew.  The food is there, but it will not be eaten with pleasure and enthusiasm--it may not even be eaten at all--because it lacks flavor.
2. By the development which fulfills a natural potential, as when living things mature from their seed.
3. By a new version of the same thing.  Henry Ford’s model T, for example, might be said to be perfected in the modern automobile.

Jesus said that he came to perfect the commandments. Which of these three models of perfection did he have in mind?

Catholic tradition has generally assumed that Jesus had the third model in mind: Catholicism is a new model of Divine worship—a new religion-- which has taken the place of the old, much as modern automobiles have displaced the model T.  In keeping with that, you explained that the sacrifice of the Mass was superior in every way to the sacrificial service of the Law of Moses, and avoided any suggestion that it could be used by a Catholic Jew to deepen his Catholic faith.

But it seems to me that Jesus explicitly excludes the third model when he adds that he did not come to abrogate the commandments. What he meant was:  I have come to perfect the commandments, not by giving you a new set of commandments in place of the old, not by giving you a late model Chevy in place of your model T, but by giving you something new which renews the old and makes it perfect. What He gave—what G-d gave through His Son--was a power for salvation, a grace, which animates Christ’s Jewish faithful with a new love for the commandments and perfects their observance of them, so that they keep them with an intense awareness of spiritual perfection they suggest. Through His Son, G-d gave us something new (the New Testament) to combine with what we already had (the Old Testament) to bring it to perfection.  That’s the first model. And then, Catholic tradition, beginning with the Gospel writers themselves (especially Matthew) affirms that this new element is the realization, the full maturity, of the Covenant G-d made with Israel. Of course, this is not an instance of natural growth, but rather the realization of Divine intentions manifest in the Old Testament itself when it is read from the point of view of Christian faith. That’s the second model.   

It seems to me that a Christian cannot help but be a supersessionist, i.e., believe that the religion of the New Testament has replaced--taken the place of--the religion of the Old Testament, that the Church, as the new People of G-d, has replaced and taken the place of the Jewish People, unless he rejects interpreting the New Testament according to the third model of perfection. And he can’t do that until he looks at Judaism in a new way: not, as so often in the past, with the purpose of demonstrating the gap between Judaism and Catholicism, but rather, with a view to acknowledging the continuity and similarities which allow the external forms of Jewish observance to be applied toward the realization of Catholic spiritual ideals.