Oaths rarely necessitated a theological belief, but rather were predicated on it.
That is, excommunication of a ruler exempted his vassals from a religious duty to fulfill their oaths.
Religious freedom existed in some limited fashion for urban minorities (like Jews), and existed in the short-run for many sects (like the Cathars), but, in the long run... not so much (see: Albigensian Crusade).
Notably, it is not until the reformation that much in the way of "formal" religious policy gets addressed in Western Europe. Byzantine states had some discussions, and the dispute over the conversion of Serbia and Bulgaria between the Roman and Greek churches might make a productive investigation.
However, it isn't until the Peace of Augsburg that the uppermost lords of the Holy Roman Empire were permitted to adhere to any faith other than Roman Catholicism. Then again, there weren't tons of options on the table at the time either.
