Religion in Western Europe 600 - 1600
In the last years of the Roman Empire Western Europe was a patchwork of belief systems, most notable among them the pagan cult of Wotan, the remnants of the Celtic druids and Christianity. Having won the conversion of Emperor Constatine some years earlier European Christianity was centred in Rome, and later gained a foothold in Ireland. Its spread was gradual and by the 10th century Christianity dominated Europe.
Religion in the SCA
Sometime during its history, the SCA decided to ban all overt use of religion. This law was enacted because incidents showed some kind of ruling was required (see sidenote). While it does dispose of a very important part of medieval times, it also takes away many of the bad aspects. (e.g. it's not fun to be on the wrong side of a Crusader, a Catholic in Reformation England, etc).
In general, a guideline is to not inflict religion on anyone else while attending an SCA event, and everyone will get along fine. Many people adopt personas that are deeply religious, because their medieval counterpart would have been deeply religious. The problem is only with inflicting your religious beliefs (even if they're only "pretend" ones) on other people who might find that offensive. (And some people might be offended if you pretend to hold religious beliefs or positions you don't actually hold; this is the actual problem with monk or nun personas, assuming that actual monks or nuns are not likely to join the SCA.) Feel free to carry or use a rosary or other religious artifact; just don't rub other people's faces in it, and don't force them (or appear to force them) to be a part of your religious observance.