Priests allowed, of course, but not Favoured Souls because - well, they're not deities.
It would also make sense for those Hellfire Warlocks to have Mephistopheles as their patron, too!
And who wouldn't want to see a Chosen of Graz'zt?
But seriously, I believe it's something quick to add and that might help cultists either.
I don't know, What do you think?