Of course there is a long Christian history of anti-Judaism. Since it wasn't racist in nature, it probably serves clarity to call it something besides anti-Semitism, but that doesn't make it good. The way I think it came about was this: Christians and Jews originally competed in an open market in the Roman world. In that competition, Christians developed a set of attitudes and some bad theology that lingered when the situation was quite different.
Christians have not always been violent towards Jews, but the situation began to get bad around the beginning of the second millenium, when a considerable amount of control-freakery took hold in both the church and the new nation-states. Moreover it took Christianity a long time to figure out that its mission could not be helped along by coercive state power. All this added up to a corruption which led Christians to ignore the fundamental commandments of Jesus in their treatment of Jews. Our record overwhelmingly stinks.
Things got yet worse in the 19th century when liberal theology denied that Christianity had any important roots in Judaism at all. All the big-name German liberal theologians in the 19th century denied that the Jewish Scriptures were God's word at all. This permitted an even more intensely and comprehensively negative attitude to Jews. The rise of romantic nationalism and pseudo-biological racialism gave secularized Christian anti-Jewish attitudes new and more extreme avenues of expression.
Some small changes for the better also began in the 19th century. As a kind of foreshadowing of the relationship emerging between Israel and evangelicals, some conservative biblicists were rethinking the doctrinal basis for Christian anti-Judaism. Ironically, the pietistic societies for missions to the Jews were often the most determined enemies of anti-Semitism. But all this didn't reach very far into the general Christian consciousness.
In the Hitler period, the churches inside and outside Germany mostly did badly. The handful of real martyrs and confessors does not make up for the sluggish indifference or collaboration of the rest. But the phenomenon of Nazism gave the impetus for the serious thinkers in all the western Churches to reconsider in a deep-going way the traditions that had contributed to the horror. It is not anything we can be puffed up about, but today most western Christians reject the old teachings of contempt. It is the bondage of liberal Christianity to the left, more than any survival of the old teachings, that causes so many liberal Christians to fawn on the enemies of Israel.
The point of this new Hitler evidence should not to assuage the Christian conscience but to force on us the realization the Church is so deeply rooted in Israel that anti-Semitism (even on the part of Christians) is always by its own logic going to turn against Christianity. We can't untangle our fate from yours. It is shameful that Hitler saw this indissoluble bond more clearly than we did. Perhaps in the third millennium we will do better.
Very thoughtful post. My personal take on it is unlike anything I've ever read anywhere. Up until the conversion of Constantine in around 300 AD, when the church became Rome's official religion, Christians were more or less a denomination of the Jewish religion, with Jews being sometimes in leadership roles. From Durant's work, I gather that there were Jewish-Christian congregations all over the Roman world.
But when Rome became officially Christian, I suspect that the schism (sp?) happened. Why? Here comes my own unique version of events: Because the Jews expected to be allowed back into Jerusalem, from which they had been banished in roughly 70AD after yet another rebellion against Rome. They must have raised hell over the issue, from within the church, and as a result the Romans tossed them out of the church. It may be that some of the gospels were very slightly edited to emphasize the split. Ever since, the Jews have been "out". Of course, Rome is pretty much out of it too, so the reasons for the split have long ago vanished. But these things have a momentum of their own.