Theurgy is a form of magic received as a gift from the gods. This power tends to only arise among worshippers of the larger, western gods such as the Three or the One Sun. Worshippers of the Kami of the Silverstars tend to have their power manifest in the form of Shugendo.
As Theurgy is a gift from a god (or gods) to its worshippers, the powers granted tend to help the survival of a practitioner. Some example powers that it may grant are the creation of food and water to feed a congregation, healing the wounds and sicknesses of a person who is suffering, or trusting the god to ward off the slings and arrows of his foes. Of course, the powers granted depend dramatically on the god in question - the god of war would obviously grant powers of a militaristic nature, both defensive and offensive, whereas tales exist of followers of Elona who can grant purest ecstasy with a single touch. A theurgist who received powers from a range of gods, such as the Three or the Four Brothers, would be truly formidable indeed.