Holy Orders
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 word order in Roman antiquity designated an established civil body, especially a governing body. Ordinatio means incorporation into an ordo. In the Church there are established bodies which Tradition, not without a basis in Sacred Scripture, has since ancient times called taxeis (Greek) or ordines. And so the liturgy speaks of the ordo episcoporum, the ordo presbyterorum, the ordo diaconorum. Other groups also receive this name of ordo: catechumens, virgins, spouses, widows,. . . .
Integration into one of these bodies in the Church was accomplished by a rite called ordinatio, a religious and liturgical act which was a consecration, a blessing or a sacrament. Today the word "ordination" is reserved for the sacramental act which integrates a man into the order of bishops, presbyters, or deacons, and goes beyond a simple election, designation, delegation, or institution by the community, for it confers a gift of the Holy Spirit that permits the exercise of a "sacred power" (sacra potestas) which can come only from Christ himself through his Church. Ordination is also called consecratio, for it is a setting apart and an investiture by Christ himself for his Church. The laying on of hands by the bishop, with the consecratory prayer, constitutes the visible sign of this ordination.
The chosen people was constituted by God as "a kingdom of priests and a holy nation." But within the people of Israel, God chose one of the twelve tribes, that of Levi, and set it apart for liturgical service; God himself is its inheritance. A special rite consecrated the beginnings of the priesthood of the Old Covenant. The priests are "appointed to act on behalf of men in relation to God, to offer gifts and sacrifices for sins."
Instituted to proclaim the Word of God and to restore communion with God by sacrifices and prayer, this priesthood nevertheless remains powerless to bring about salvation, needing to repeat its sacrifices ceaselessly and being unable to achieve a definitive sanctification, which only the sacrifice of Christ would accomplish.
* (All references taken from the Catechism of the Catholic Church, #1536-1540)