In a perfect world, I'm not even sure we would have immortal patrons of cabals with religions that weren't in absolute sync with their cabals. However, as anyone who's been flagged (along with 60 other people) by a rogue Arbiter who thinks he's being original, then unflagged by Bria (who has a religion that in my opinion could read Chaotic Good at least as easily as Lawful Good) can tell you, it's generally good for the well-being and smooth running of CF that they wear two hats rather than be uncaballed.

I look at it something like this: As an immortal, your religion says something about the things that are important to you (IC) and the way you think the world should be. Your cabal may point to one way of going about that goal, but it's not necessarily the only way. If Ilraeth thinks Knowledge is Good (forgive the gross religion oversimplification), since Warlocks like knowledge (well, in theory), watching over them and helping them out may be one way to promote it. Maybe whatever Armand is doing is another way. I think it makes for a somewhat more interesting game that not all immortals will come down and say "this is the way."

To dip into the dangerous mire of real-world religious examples, I don't think you'd have to look very hard in history to find incidents where one faction of a religion went to war with another, possibly very similar, faction of the same religion. (Theirs is the right one and the other one is wrong, of course. :p) You could play it out very similarly in the game. Mage followers of Ilraeth could possibly see an Armand as a radical heretical crack-monkey to be burned (frozen?) at the stake, and vice-versa.