Has anyone noticed a pattern about your alliances choosing sides when you are involved in conflict with a faction that they are also allied with? I mean, what causes the ai to choose your side over the other? Generally, once I start to control more of the provinces on the map, nobody stays allied with me, even if I am a Christian faction attacing a non-Christian faction.

I am currently enjoying a MedMod 3.12 campaign, and I was attacked on the same turn by both the Swedes (Christian) and the Russians (Orthodox). I had a large number of alliances on the previous turn, but the only ones who stuck by me were the Danish. In fact the Pope reprimanded me (playing as the Poles) even though it was MY ships getting sunk by the Swedes. Basically, since I wasn't expecting the Swedish attack, I was still moving my armies up to get to them - and the Pope just made me take a 15 yr long time-out.

But among the cancelled aliiance messages, a number were from Christian factions that were choosing to stay aligned with the Russians. So religion may play a part, but it is not always a deciding factor.