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Catholic Church

Ordained clergy in the Catholic Church are either deacons, priests, or bishops belonging to the diaconate, the presbyterate, or the episcopate, respectively. Among bishops, some are metropolitans, archbishops, or patriarchs. The pope is the bishop of Rome, the supreme and universal hierarch of the Church, and his authorization is now required for the ordination of all Roman Catholic bishops. With rare exceptions, cardinals are bishops, although it was not always so; formerly, some cardinals were people who had received clerical tonsure, but not Holy Orders. Secular clergy are ministers, such as deacons and priests, who do not belong to a religious institute and live in the world at large, rather than a religious institute (saeculum). The Holy See supports the activity of its clergy by the Congregation for the Clergy (), a dicastery of Roman curia.

Canon Law indicates (canon 207) that "y divine institution, there are among the Christian faithful in the Church sacred ministers who in law are also called clerics; the other members of the Christian faithful are called lay persons". This distinction of a separate ministry was formed in the early times of Christianity; one early source reflecting this distinction, with the three ranks or orders of bishop, priest and deacon, is the writings of Saint Ignatius of Antioch.

Holy Orders is one of the Seven Sacraments, enumerated at the Council of Trent, that the Magisterium considers to be of divine institution. In the Catholic Church, only men are permitted to be clerics.

In the Latin Church before 1972, tonsure admitted someone to the clerical state, after which he could receive the four minor orders (ostiary, lectorate, order of exorcists, order of acolytes) and then the major orders of subdiaconate, diaconate, presbyterate, and finally the episcopate, which according to Roman Catholic doctrine is "the fullness of Holy Orders". Since 1972 the minor orders and the subdiaconate have been replaced by lay ministries and clerical tonsure no longer takes place, except in some Traditionalist Catholic groups, and the clerical state is acquired, even in those groups, by Holy Orders. In the Latin Church the initial level of the three ranks of Holy Orders is that of the diaconate. In addition to these three orders of clerics, some Eastern Catholic, or "Uniate", Churches have what are called "minor clerics".

Members of institutes of consecrated life and societies of apostolic life are clerics only if they have received Holy Orders. Thus, unordained monks, friars, nuns, and religious brothers and sisters are not part of the clergy.

The Code of Canon Law and the Code of Canons of the Eastern Churches prescribe that every cleric must be enrolled or " incardinated " in a diocese or its equivalent (an apostolic vicariate, territorial abbey, personal prelature, etc.) or in a religious institute, society of apostolic life or secular institute. The need for this requirement arose because of the trouble caused from the earliest years of the Church by unattached or vagrant clergy subject to no ecclesiastical authority and often causing scandal wherever they went.

Current canon law prescribes that to be ordained a priest, an education is required of two years of philosophy and four of theology, including study of dogmatic and moral theology, the Holy Scriptures, and canon law have to be studied within a seminary or an ecclesiastical faculty at a university.

Clerical celibacy is a requirement for almost all clergy in the predominant Latin Church, with the exception of deacons who do not intend to become priests. Exceptions are sometimes admitted for ordination to transitional diaconate and priesthood on a case-by-case basis for married clergymen of other churches or communities who become Catholics, but consecration of already married men as bishops is excluded in both the Latin and Eastern Catholic Churches (see personal ordinariate). Clerical marriage is not allowed and therefore, if those for whom in some particular Church celibacy is optional (such as permanent deacons in the Latin Church) wish to marry, they must do so before ordination. Eastern Catholic Churches while allowing married men to be ordained, do not allow clerical marriage after ordination: their parish priests are often married, but must marry before being ordained to the priesthood. Eastern Catholic Churches require celibacy only for bishops.