The Cult of Avacyn
The world of Innistrad is full of vampires, zombies, werewolves other monsters in such proliferation that it should not be possible for humans to exist in it. The last human would have been torn to pieces by a werewolf or drained of blood by a vampire a long time ago were it not for the vampire planeswalker Sorin Markov.
Sorin created an artificial being called Avacyn and gave her the form of an Angel. Avacyn was given the task of defending humanity and enough power to counter even the strongest denizens of Innistrad - the demons. But Avacyn's ability to defend the humans of Innistrad goes well beyond her personal power, she is able to grant humans power to counter the darkness as long as they believe in her.
Through some means known only to Soren Avacyn is able to impart power to people who believe in her. At the center of her worship is an organized church.
Member's of Avacyn's church use their power both to fight and to heal.
The church is headed by a Lunarch. At the time we look into Innistrad, the Lunarch is Mikaeus. The second card shows Mikaeus has a dark fate awaiting him.
Avacyn also has her sigil or symbol.
Avacyn's symbol is no mere decoration but a means of harnessing Avacyn's power. The use of this holy sign is not exclusive to the clergy. It is a common item of protection among the people of Innistrad. The card "Travel Preparations" show the symbol in daily use as a tool for protection. And "Avacyn's Pilgrim" is shown carrying the sigil on a staff.
Both cards show travel as a particular reason to seek out Avacyn's protection. Understandably so, since for humans, Innistrad is full of threats.
As the Eldrazi Emrakul's influence grew over Innistrad, some Church members became part of the newly formed cult of Emrakul.
Emrakul's power corrupted not only Avacyn's followers but Avacyn herself. From a benigh protrectress she became a despotic, and bloody dictator. Her church, took on this new and brutal facade - changing from kindly priests to enforcers.
As Emrakul's power waxed, well, the tranformation of the former Avacinian church officials became rather more than a change of robes.
Not all of the church of Avacyn's members became corrupted though.