The chosen people was constituted by God as "a kingdom of priests and a holy nation." 6 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. 7 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." 8

Everything that the priesthood of the Old Covenant prefigured finds its fulfillment in Christ Jesus, the "one mediator between God and men." 15 The Christian tradition considers Melchizedek, "priest of God Most High," as a prefiguration of the Priesthood Of Christ, the unique "high priest after the order of Melchizedek"; 16 "holy, blameless, unstained," 17 "by a single offering he has perfected for all time those who are sanctified," 18 that is, by the unique sacrifice of the cross.

The ministerial or hierarchical priesthood of bishops and priests, and the common priesthood of all the faithful participate, "each in its own proper way, in the one Priesthood Of Christ." While being "ordered one to another," they differ essentially. 22 In what sense? While the common priesthood of the faithful is exercised by the unfolding of baptismal grace - a life of faith, hope, and charity, a life according to the Spirit - ,the ministerial priesthood is at the service of the common priesthood. It is directed at the unfolding of the baptismal grace of all Christians. the ministerial priesthood is a means by which Christ unceasingly builds up and leads his Church. For this reason it is transmitted by its own sacrament, the sacrament of Holy Orders.

"The divinely instituted ecclesiastical ministry is exercised in different degrees by those who even from ancient times have been called bishops, priests, and deacons." 32 Catholic doctrine, expressed in the liturgy, the Magisterium, and the constant practice of the Church, recognizes that 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. For this reason the term sacerdos in current usage denotes bishops and priests but not deacons. Yet Catholic doctrine teaches that the degrees of priestly participation (episcopate and presbyterate) and the degree of service (diaconate) are all three conferred by a sacramental act called "ordination," that is, by the sacrament of Holy Orders: