A priest or priestess, is a religious leader authorized to perform the sacred rituals of a religion, especially as a mediatory agent between humans and one or more deities.
Religion is a cultural system of behaviors and practices, world views, sacred texts, holy places, ethics, and societal organisation that relate humanity to what an anthropologist has called "an order of existence".
A ritual "is a sequence of activities involving gestures, words, and objects, performed in a sequestered place, and performed according to set sequence."
Clergy are some of the main and important formal leaders within certain religions.
The Priests - Ave Maria by ThePriestsVEVO
They also have the authority or power to administer religious rites; in particular, rites of sacrifice to, and propitiation of, a deity or deities.
Sacrifice is the offering of food, objects or the lives of animals to a higher purpose, in particular divine beings, as an act of propitiation or worship.
A rite or ritual is an established, ceremonial, usually religious, act.
A deity is a supernatural being considered divine or sacred.
The Priests - O Holy Night by ThePriestsVEVO
Their office or position is the priesthood, a term which also may apply to such persons collectively.