Theurgy represents the White Magic of the Universal Church as well as the other faiths of the setting. GMs should make efforts to define the rarity and how theurgy works in their games. From a game-mechanic point of view, anyone can use theurgy, even non-priests. Its just a set of rules. In-character though, theurgy may be common among most priests for a high-fantasy feel, or taught secretively be occult movements within the various sects.