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 from official events. 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. Personal/Household expressions of religion are acceptable, but religious activities may not be an official part of any SCA event. 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. While many in the SCA enjoy playing a Monk or Nun persona one must not display bigotry by portraying it with satire.) Feel free to carry or wear a rosary/paternoster or other religious artifact or costume;, just don't force them (or appear to force them) to be a part of your religious observance. If you are part part of a religious household/order/other you may hold religious ceremonies in your private tent or groups camp (if all attending agree) but use common sense, no dancing naked around a bonfire for all to see. Discretion and politeness is the key.
Curiously, many groups turn a blind eye to the practice of venerating relics or swearing oaths by certain saint, so long as those relics and oaths are based on fictional SCA saints like Saint Cunard the Tenacious. Many households and individuals can and do use real saints though but only as individuals and most often the relic is a mock up to look like a real one (Clay fingers, Plastic skulls, etc) for obvious reasons.