OK, so we know that dead people who have had a christian burial cannot be contacted via necromancy.
Is this rule written anywhere except in the description of some spell effects?
And what exactly does it stop?
If someone has received a christian burial can they come back as a ghost anyway (say if they were cruelly murdered and their killer has married their sweetheart, classic)? And if so are they immune to necromancy and would this include spells that affect the ghost if it is present or only spells to summon it?
My take is that christian burial protects people from having their ghosts called forth because the ghost generally doesn't exist. But if a person with such a burial DOES come back as a ghost despite the burial then it is susceptible to hermetic magic, and this includes summoning and binding. This weakness to hermetic magic disappears if the ghost finds peace of course since the ghost no longer exists.
I say it is one more rule which is prejudicial to non-Christians and their funerary arrangements, and tend to ignore it. Seriously the amount of religious predjudice in this game is stomach turning.
Realms of Power:Magic p117 "Where do Ghosts come from?" sidebar suggests it's up to you whether or not religious rites lay ghosts to rest or not. I think that section of RoP:M is the best bit on ghosts in the whole line.
As for Christian burial being privileged - to be fair, other monotheist faiths (Islam, Judaism, Zoroastrianism) get the Divine special treatment. The Mythic Europe of Ars is frequently less brutal than historical Europe with its crusades, pogroms and punishment of heretics. In fact, most RPGs tend to avoid brutalities on a par with 2017 Syria.
EDIT:Ghosts in Ars Magica (p72 of TMRE, reprinted from Calebais) doesn't mention anything about burial.
Canaanite Necromany in Ancient Magic does have something to say - it says the immunity to necromancy granted by extreme unction (See RoP:D page 75) is only a flaw in Hermetic theory, which is corrected by integrating Canaanite necromancy. Therefore, see RoP:D page 75 - time for me to dig out my copy!
2nd Edit: RoP:D p111 confirms Muslim burial prevents ghosts from being summoned. RoP:D p75 says Extreme Unction prevents someone from being affected by any supernatural power for three days, and that the spirit of a person given a Christian burial can never be affected by an kind of necromany or magic, though the body can be if it is physically removed from the churchyard. I think the Grogs book also includes details on people who survive the three day period following Extreme Unction.