An interesting question, answered by David Galernter

Why should a Jew care whether Christianity lives or dies?

It comes down to this: Christianity is the Jews’ gift to mankind; the most important gift mankind has ever received. That so many modern leftists would say to themselves, 'All the more reason to hate the Jews,' merely underlines the point. The natural enemy of the Jew is the natural enemy of the Christian, too—the conscience-hater, the man who wants no witnesses. Why should a Jew care whether Christianity lives or dies? Because he must care whether the message of Judaism lives or dies, whether the mission of Judaism fails or succeeds.In the end, that hardly matters. The important question is not why a Jew, but why a human being should care about the fate of Christianity.
And the answer is exactly the same.