Mythic Europe is the setting for Ars Magica, where the world of medieval europe is happening, but the myths and legends are true. The following is general knowledge known by the magi.
The Levant Tribunal
Little is known about this far Tribunal.
The Rhine Tribunal
The first Covenant of the Order and home of the Great Library.
The home of House Bjornaer, and the site of that house's Gathering of Twelve Years. The current Primus is Urgen Twinslayer, the great bear.
The Roman Tribunal
The home of House Mercere,the center of its activities.
The most important consideration in all of this is that the Saga itself and the characters within are works of fiction. Similarity to living or dead persons or places is entirely intended, as the Saga is set in a fantasy version of Europe. However, neither character judgement nor historical accuracy are key - and as such some persons, places, events, characters, philosophies and anything else you may think of may and almost definitely will vary from the real world.
It's all make-believe. Anyone who wants to pretend it is anything else is being silly.
The major interpretation that the GM has placed upon this game is that regardless of modern sensibilities regarding 'The Church', in Mythic Europe the Church is made up of people.
'The Church' is not an evil entity devoted to hunting wizards but is made up of thousands of clergymen who each have their own goals and wants. While some clergymen may well despise magi, others equally are intrigued or simply disinterested.
In the fictional setting of Mythic Europe the power of God is without equal. This statement makes no claim either way to beliefs or the faith of the players in the real world, and the group itself is comprised of people of a variety of faiths and beliefs.
There is a small tolerance of characters who want to believe that the Divine is not all-powerful. Players must accept that in Mythic Europe these characters are simply wrong. Mythic Europe is set in a fictitious variant of the 13th Century, where the inhabitants have (mostly) medieval assumptions about the world.
Faeries are odd creatures made of stories. Each person may have their own theory as to what Faeries are, but the Faeries aren't telling.
Magic is the world of perfect ideas. Magic is the process of making those perfect forms react upon the world. Magical creatures are more 'perfect' creatures that exist within the world, because the world is sometimes more or less perfect.
Magic in itself is not inherently ungodly, as God is the creator and thus created magic. What a person does with magic determines whether it is used for good or evil.
The infernal are creatures of lies, foulness, and cruelty. Their purpose is to tempt souls into sin, which then they use to tempt more people into sin. For the most part, the plans of the Infernal are not ever-present. Large organisations are not inherently evil, and the actions of such organisations are judged according to Medieval standards, not modern ones.
The agents of the Infernal are always villains, and always absolutely evil. The Infernal is always the enemy within the Saga, never anything else.