India, anyone??
Monotheistic North, Polytheistic South?
Though, of course, I know little of Indian history, so I don't know how long this state of affairs has been going on. But the fact is, it can happen.
EDIT: A lot of countries were Christianized because of issues of trade. This is because the richest parts of Europe, the former possessions of the Roman Empire, were Christianized first, so if anyone in Northern or Eastern Europe wanted to get in on that, they had to become Christian, because Christians couldn't trade with Non-Christians (same thing happened with Islam). Now, what I assume happened in India was that the Polytheists in the South had no pressing reason to convert; i.e., the Monotheists in the North had nothing they wanted, OR were willing to trade with them. So all we need are a few rich, non-Christian countries in Europe (hard to do because of the way trade worked) and then you can have a significant portion of Europe non-Christian.
Crusades, etc, which come along later, are a different matter, I suppose.
EDIT EDIT: But if the Catholics are Crusading against heathens in Europe, then that means they can't (probably) go Crusading against Muslims in the Middle East, which has all sorts of long-term consequences. Then again, the whole reason for the Crusades was to unite Christianity and keep Christians from fighting one another, so there would probably be no need for Crusades at all ITTL, since Christians would be fighting each other a little bit, but they would be fighting Non-Christians even more.
Also, if the Byzantine Emperor still asks the Pope for help, maybe he'll actually get the help he wanted in the form of Latin mercenaries, rather than a huge occupying force that largely ignored the Seljuks. So, ITTL, we could also have a resurgent Byzantine Empire, always a plus.