The Anarchy of King Stephen
The Roman Catholic church is by far the dominant religion in Christendom. It controls and influences virtually every aspect of everyone’s lives. Local parish priests are “supervised” by bishops, who in turn owe obeisance to the Pope in Rome.
Judaism, Islam and the Byzantine Orthodox church are not unknown — the Jews are looked upon with suspicion, the Muslims are viewed as heretics and enemies, and an Orthodox believer might be treated as just another Christian by the peasantry.
The pagan beliefs of the Norse were historically extinct by 1100 AD, but in the Kingdom of Wessex campaign setting, their conversion is not complete. Holdouts of the Old Ways exist in the remote places of Scandanavia, and Iceland remains unconverted.
The ancient beliefs are not completely dead — in more organized times, the Church does their best to keep a lid on heresy and the practice of hedge witchery and Celtic folk religion, but now, during the Anarchy, they simply do not have the strength of arms to enforce such decrees. The old ways are still kept hidden, with their practitioners dutifully attending Mass on Sunday, but virtually every village will have a wise elder or three who everyone knows to turn to for a charm, a potion, or a curse.