CIC 1548 In the ecclesial service of the ordained minister, it is Christ himself who is present to his Church as Head of his Body, Shepherd of his flock, high priest of the redemptive sacrifice, Teacher of Truth. This is what the Church means by saying that the priest, by virtue of the sacrament of Holy Orders, acts in persona Christi Capitis:1
It is the same priest, Christ Jesus, whose sacred person his minister truly represents. Now the minister, by reason of the sacerdotal consecration which he has received, is truly made like to the high priest and possesses the authority to act in the power and place of the person of Christ himself (virtute ac persona ipsius Christi).2
Christ is the source of all priesthood: the priest of the old law was a figure of Christ, and the priest of the new law acts in the person of Christ.3
1 Cf. LG 10; 28; SC 33; CD 11; PO 2; 6.
2 Pius XII, encyclical, Mediator Dei: AAS, 39 (1947) 548.
3 St. Thomas Aquinas, STh III,22,4c.