The Church adopted the term order from its use in the Roman Empire, where it referred to a governing group. In the Sacrament of Holy Orders, there are three degrees or "orders": bishop, priest, and deacon. The rite of ordination is the sacramental act that makes this possible. Ordination "confers a gift of the Holy Spirit that permits the exercise of a 'sacred power' . . . which can come only from Christ himself through the Church" (CCC, no. 1538).
The first priest figure to appear in the Old Testament is Melchizedek, who offered a sacrifice of bread and wine on behalf of the patriarch Abraham (Gn 14:18-20). He symbolized the permanence of priesthood: "Like Melchizedek you are a priest forever" (Ps 110:4). God also chose Aaron and his sons to be priests (Ex 28:1ff.) and designated the tribe of Levi for liturgical service. They acted on behalf of the people and offered gifts and sacrifices for sins. They proclaimed God's Word and led people to communion with him through sacrifices and prayers.
But these priests were unable to provide the fullness of salvation or definitive sanctification for the people. Only the sacrifice of Jesus Christ could bring this about. The priesthood of Melchizedek, Aaron, and the Levites prefigured the priesthood of Christ, as is seen in consecration prayers for the ordination of bishops, priests, and deacons.
The priesthood of the Old Testament found its perfect fulfillment in the priesthood of Jesus Christ, who is the one mediator between God and us. Jesus' sacrifice of himself on the Cross was a priestly act of perfect self-offering accepted by the Father and culminating in his Resurrection from the dead so that, as Risen Lord and High Priest, he continues to offer salvation to all.
By Baptism, all the members of the Church share in Christ's holy priesthood. It is called "the common priesthood of the faithful" because the entire Church shares in it. To build up this priesthood, Christ gives to his Church the ordained ministries of bishops, priests, and deacons through the Sacrament of Holy Orders. Only the ordained bishop and priest may be ministers of Confirmation (or Chrismation), the Eucharist, the Sacrament of Penance and Reconciliation, and the Sacrament of the Anointing of the Sick. Only bishops may ordain deacons, priests, and other bishops. "The ministerial priesthood differs in essence from the common priesthood of the faithful because it confers a sacred power for the service of the faithful. The ordained ministers exercise their service for the People of God by teaching (munus docendi), divine worship (munus liturgicum) and pastoral governance (munus regendi)" (CCC, no. 1592). Deacons in the Latin Church can baptize and witness the Sacrament of Marriage, as do priests and bishops.
The ordained bishop and priest serve the Church in the person of Christ as head of the Body. "Through the ordained ministry, especially that of bishops and priests, the presence of Christ as head of the Church is made visible in the midst of the community of believers" (CCC, no. 1549). The Sacrament does not preserve the ordained from weakness and sin, but the Holy Spirit guarantees that the minister's sin does not impede the effectiveness of the Sacrament and its graces. The ordained are called to a holiness of life and an attitude of humility that conforms them to Christ whose priesthood they share. The priest acts not only in the person of Christ, the Head of the Church, but also in the name of the Church when presenting to God the prayer of the Church, especially in the Eucharist.
Let everyone revere the deacons as Jesus Christ, the bishop as image of the Father, and the presbyters as the senate of God and the assembly of the apostles. For without them, one cannot speak of the Church.
-CCC, no. 1554, citing St. Ignatius of Antioch, Ad. Trail. 3, 1
By ordination to the episcopacy, bishops receive the fullness of the Sacrament of Holy Orders and become successors of the Apostles. Through this Sacrament, a bishop belongs to the college of bishops and serves as the visible head or pastor of the local church entrusted to his care. As a college, the bishops have care and concern for the apostolic mission of all the churches in union with and under the authority of the Pope-the head of the college of bishops, the Bishop of Rome, and the successor of St. Peter.
By ordination, "priests are united with the bishops in [priestly] dignity and at the same time depend on them in the exercise of their pastoral functions; they are called to be the bishops' prudent co-workers" (CCC, no. 1595). With the bishop, priests form a presbyteral (priestly) community and assume with him the pastoral mission for a particular parish. The bishop appoints priests to the pastoral care of parishes and to other diocesan ministries. The priest promises obedience to the bishop in service to God's people.
The title deacon comes from the Greek word diakonia meaning "servant." A deacon has a special attachment to the bishop in the tasks of service and is configured to Christ, the Deacon-or Servant-of all (d. CCC, nos. 1569-1570).
"There are two degrees of ministerial participation in the priesthood of Christ: the episcopacy and the presbyterate. The diaconate is intended to help and serve them" (CCC, no. 1554). The three degrees of the Sacrament of Holy Orders-bishop, priest, and deacon-are all conferred by ordination.
Deacons receive the Sacrament of Holy Orders from a bishop and are ordained not to the ministerial priesthood but to the ministry of service. Through ordination the deacon is conformed to Christ, who came to serve, not to be served. In the Latin Church, deacons may baptize, proclaim the Gospel, preach the homily, assist the bishop or priest in -the celebration of the Eucharist, assist at and bless marriages, and preside at funerals. They dedicate themselves to charitable endeavors, which was their ministerial role in New Testament times.
Whether they are involved in the Church's liturgical or pastoral life or in her social and charitable endeavors, deacons are "strengthened by the imposition of hands that has come down from the apostles. They would be more closely bound to the altar and their ministry would be made more fruitful through the sacramental grace of the diaconate" (AG, 16, no. 6).
Since the Second Vatican Council, the Latin Church has restored the diaconate as a permanent rank of the hierarchy. Now, diaconate as a permanent office may also be conferred on both married and unmarried men. The Eastern Churches have always retained it. Seminarians preparing for priesthood have always been ordained to the diaconate before ordination to priesthood.