Okay, so perhaps the devs can explain it better, but as far as I can see: the schism lets each sovereign ruler choose whether they will want to go Catholic or Orthodox when the moment of schism officially comes, along with vassals (who however are likely to follow their liege). Priests get switched to Catholic/Orthodox depending on whether they got the "Greek rites" or "Latin rites" trait before. Players can choose freely. As for what the AI chooses, that seems to be roughly laid out so that western rulers convert to Catholic, and more eastern rulers to Orthodox most of the time. But even though this is the trend, there are exceptions (basically, culture, location, religious head allegiance and a few other assorted factors are weighed to give the AI a certain probability to choose either path - being located in the Syria area seems to weigh the choice towards going Orthodox). I've run one 867 game on 4.06.8, and in that one, Brittany went Orthodox, for example (at least initially - Catholics eventually took over), while in a much older game I remember getting a Catholic Georgia (or maybe it was just an Abkhazia or something?). As far as I can tell from the event files, province religion mostly just follows the province owner's choice, or if it's owned by a non-Chalcedonian, it will just flip depending on the region of the world in a deterministic manner. So any Syrian Christians aren't extraordinarily likely to become Catholic, but there's a chance nonetheless (I haven't analyzed all the AI factors in depth). The way it looks to me, the provinces that are owned by non-Chalcedonians in the Syria region will all flip to Orthodox though. Either way, some realms can turn into a bit of a mess with the schism, with a strong secret society being formed out of the religion that wasn't chosen by the liege, so it's really fun.
