[Wrana]

One more thing - communion only boosts levels of priests who are already mages. So be careful who you include as a master. The same goes for Astral spell Power of the Spheres and Crystal Shield (which casts the same spell) - it also boosts levels, but only for priest/mages.
Actually, as Sauromatia you can use Sabbath to boost holy levels: you have many Blood mages, some of which are also priests iirc. One Sabbath Master and 4+ Slaves give you Holy 3.
Another thing I didn't test - there is a Hell Power spell. It requires Blood3 and boosts all levels by 2, also giving a Hrror mark. I am not sure whether it boosts priest levels but it should - which may be useful in a pinch.
Also there are some summons and mummification, but those above are easier to do.