Today’s Catechism sections discuss the priesthood of the Old Covenant, the words used in the Liturgy of Ordination, and the Priesthood of Christ. Supporting material comes from the “Summa Theologica”.

II. The Sacrament of Holy Orders in the Economy of Salvation

The priesthood of the Old Covenant

1539 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

1540 Instituted to proclaim the Word of God and to restore communion with God by sacrifices and prayer,9 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.10

1541 The liturgy of the Church, however, sees in the priesthood of Aaron and the service of the Levites, as in the institution of the seventy elders,11 a prefiguring of the ordained ministry of the New Covenant. Thus in the Latin Rite the Church prays in the consecratory preface of the ordination of bishops:

God the Father of our Lord Jesus Christ,
by your gracious word
you have established the plan of your Church.
 
From the beginning,
you chose the descendants of Abraham to be your holy nation.
You established rulers and priests
and did not leave your sanctuary without ministers to serve you....12

1542 At the ordination of priests, the Church prays:

Lord, holy Father, . . .
when you had appointed high priests to rule your people,
you chose other men next to them in rank and dignity
to be with them and to help them in their task....
 
you extended the spirit of Moses to seventy wise men....
You shared among the sons of Aaron
the fullness of their father's power.13

1543 In the consecratory prayer for ordination of deacons, the Church confesses:

Almighty God . . ..
You make the Church, Christ's body,
grow to its full stature as a new and greater temple.
You enrich it with every kind of grace
and perfect it with a diversity of members
to serve the whole body in a wonderful pattern of unity.
 
You established a threefold ministry of worship and service,
for the glory of your name.
As ministers of your tabernacle you chose the sons of Levi
and gave them your blessing as their everlasting inheritance.14

The one priesthood of Christ

1544 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.

1545 The redemptive sacrifice of Christ is unique, accomplished once for all; yet it is made present in the Eucharistic sacrifice of the Church. The same is true of the one priesthood of Christ; it is made present through the ministerial priesthood without diminishing the uniqueness of Christ's priesthood: "Only Christ is the true priest, the others being only his ministers."19

The “Summa Theologica” discusses Holy Orders in the Church. (Supplement, 34, 1).

Article 1. Whether there should be Order in the Church?

Objection 1. It would seem that there should not be Order in the Church. For Order requires subjection and preeminence. But subjection seemingly is incompatible with the liberty whereunto we are called by Christ. Therefore there should not be Order in the Church.

Objection 2. Further, he who has received an Order becomes another's superior. But in the Church everyone should deem himself lower than another (Philippians 2:3): "Let each esteem others better than themselves." Therefore Order should not be in the Church.

Objection 3. Further, we find order among the angels on account of their differing in natural and gratuitous gifts. But all men are one in nature, and it is not known who has the higher gifts of grace. Therefore Order should not be in the Church.

On the contrary, "Those things that are of God, are in order." Now the Church is of God, for He Himself built it with His blood. Therefore there ought to be Order in the Church.

Further, the state of the Church is between the state of nature and the state of glory. Now we find order in nature, in that some things are above others, and likewise in glory, as in the angels. Therefore there should be Order in the Church.

I answer that, God wished to produce His works in likeness to Himself, as far as possible, in order that they might be perfect, and that He might be known through them. Hence, that He might be portrayed in His works, not only according to what He is in Himself, but also according as He acts on others, He laid this natural law on all things, that last things should be reduced and perfected by middle things, and middle things by the first, as Dionysius says (Eccl. Hier. v). Wherefore that this beauty might not be lacking to the Church, He established Order in her so that some should deliver the sacraments to others, being thus made like to God in their own way, as co-operating with God; even as in the natural body, some members act on others.

Reply to Objection 1. The subjection of slavery is incompatible with liberty; for slavery consists in lording over others and employing them for one's own profit. Such subjection is not required in Order, whereby those who preside have to seek the salvation of their subjects and not their own profit.

Reply to Objection 2. Each one should esteem himself lower in merit, not in office; and orders are a kind of office.

Reply to Objection 3. Order among the angels does not arise from difference of nature, unless accidentally, in so far as difference of grace results in them from difference of nature. But in them it results directly from their difference in grace; because their orders regard their participation of divine things, and their communicating them in the state of glory, which is according to the measure of grace, as being the end and effect, so to speak, of grace. on the other hand, the Orders of the Church militant regard the participation in the sacraments and the communication thereof, which are the cause of grace and, in a way, precede grace; and consequently our Orders do not require sanctifying grace, but only the power to dispense the sacraments; for which reason order does not correspond to the difference of sanctifying grace, but to the difference of power.

Footnotes

6 ⇒ Ex 19:6; cf. ⇒ Isa 61:6.
7 Cf. ⇒ Num 1:48-53; ⇒ Josh 13:33.
8 ⇒ Heb 5:1; cf. ⇒ Ex 29:1-30; ⇒ Lev 8.
9 Cf. ⇒ Mal 2:7-9.
10 Cf. ⇒ Heb 5:3; ⇒ 7:27; 101-4.
11 Cf. ⇒ Num 11:24-25.
12 Roman Pontifical, Ordination of Bishops 26, Prayer of Consecration.
13 Roman Pontifical, Ordination of Priests 22, Prayer of Consecration.
14 Roman Pontifical, Ordination of Deacons 21, Prayer of Consecration.
15 ⇒ 2 Tim 2:5.
16 ⇒ Heb 5:10; cf. ⇒ 6:20; ⇒ Gen 14:18.
17 ⇒ Heb 7:26.
18 ⇒ Heb 10:14.
19 St. Thomas Aquinas, ⇒ Hebr. 8, 4.