A priest can heal your troops, villigers, and seige weapons. The priest can also convert enemy troops, villigers, seige weapons, and buildings (except for the Town Center.)
Researching Astrology allows faster conversions. Mysticism increases hit points. Polytheism increases speed. Fanaticism speeds Priest rejuvenation after conversion. Monotheism allows conversion of enemy Priests and buildings. Afterlife increases range.