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,. . . .
St. Paul said to his disciple Timothy: “I remind you to rekindle the gift of God that is within you through the laying on of my hands” (2 Tim 1:6), and “If any one aspires to the office of bishop, he desires a noble task.” (1 Tim 3:1) To Titus he said: “This is why I left you in Crete, that you amend what was defective, and appoint presbyters in every town, as I directed you” (Titus 1:5).
The whole Church is a priestly people. Through Baptism all the faithful share in the priesthood of Christ. This participation is called the “common priesthood of the faithful.” Based on this common priesthood and ordered to its service, there exists another participation in the mission of Christ: the ministry conferred by the sacrament of Holy Orders, where the task is to serve in the name and in the person of Christ the Head in the midst of the community.