Holy Orders is the sacrament through which the mission entrusted by Christ to his apostles continues to be exercised in the Church until the end of time: thus it is the sacrament of apostolic ministry. It includes three degrees: episcopate, presbyterate, and diaconate.
The Sacrament of Holy Orders is the continuation of Christ’s priesthood, which He bestowed upon His Apostles; thus, the Catechism of the Catholic Church refers to the Sacrament of Holy Orders as “the sacrament of apostolic ministry.”
“Ordination” comes from the Latin word ordinatio, which means to incorporate someone into an order. In the Sacrament of Holy Orders, a man is incorporated into the priesthood of Christ, at one of three levels: the episcopate, the priesthood, or the diaconate.
The priesthood was established by God among the Israelites during their exodus from Egypt. God chose the tribe of Levi as priests for the nation. Their primary duties were the offering of sacrifice and prayer for the people.
Christ, in offering Himself up for the sins of all mankind, fulfilled the duties of the Old Testament priesthood once and for all. But just as the Eucharist makes that sacrifice present to us today, so the New Testament priesthood is a sharing in the eternal priesthood of Christ. While all believers are, in some sense, priests, some are set aside to serve the Church as Christ Himself did.
The Sacrament of Holy Orders can be validly conferred only on baptized men, following the example set by Christ and His Apostles, who chose only men as their successors and collaborators. A man cannot demand ordination; the Church has the authority to determine eligibility for the sacrament.
While the episcopate is reserved to unmarried men, the discipline regarding the priesthood varies in East and West. The Eastern Churches allow married men to be ordained priests, while the Western Church insists on celibacy. Once a man has received the Sacrament of Holy Orders, however, he cannot marry.
As the Catechism of the Catholic Church notes:
The essential rite of the sacrament of Holy Orders for all three degrees consists in the bishop’s imposition of hands on the head of the ordinand and in the bishop’s specific consecratory prayer asking God for the outpouring of the Holy Spirit and his gifts proper to the ministry to which the candidate is being ordained.
Other elements of the sacrament, such as holding it in the cathedral (the bishop’s own church); holding it during Mass; and celebrating it on a Sunday are traditional but not essential.
Because of his role as a successor to the Apostles, who were themselves successors to Christ, the bishop is the proper minister of the sacrament. The grace of sanctifying others that he receives at his own ordination allows him to ordain others.
The Sacrament of Holy Orders, like the Sacrament of Baptism and the Sacrament of Confirmation, can only be received once for each level of ordination. Once a man has been ordained, he is spiritually changed, which is the origin of the saying, “Once a priest, always a priest.” He can be dispensed of his obligations as a priest (or even forbidden to act as a priest); but he remains a priest forever.
Each level of ordination confers special graces, from the ability to preach, granted to deacons; to the ability to act in the person of Christ to offer the Mass, granted to priests; to a special grace of strength, granted to bishops, which allows him to teach and lead his flock, even to the point of dying as Christ did.