9 But no created power was sufficient to expiate the sins of men, if the Son of God had not assumed man's nature in order to redeem it. This, indeed, the Savior of men Himself declared by the mouth of the sacred Psalmist: "Sacrifice and oblation thou wouldest not: but a body thou hast fitted to me: Holocausts for sin did not please thee: then said I: Behold I come" ( Hebrews x, 5-7). And in very deed, "Surely He hath borne our infirmities, and carried our sorrows. . . He was wounded for our iniquities ( Isaias liii, 4-5), and He His own self bore our sins in His body upon the tree . . . (1 Peter ii, 24), "Blotting out the handwriting of the decree that was against us, which was contrary to us. And He has taken the same out of the way, fastening it to the cross . . ." ( Colossians ii, 14) "that we being dead to sins, should live to justice" (1 Peter ii, 24). Yet, though the copious redemption of Christ has abundantly forgiven us all offenses (Cf. Colossians ii, 13), nevertheless, because of that wondrous divine dispensation whereby those things that are wanting of the sufferings of Christ are to be filled up in our flesh for His body which is the Church (Cf. Colossians i, 24), to the praises and satisfactions, "which Christ in the name of sinners rendered unto God" we can also add our praises and satisfactions, and indeed it behoves us so to do. But we must ever remember that the whole virtue of the expiation depends on the one bloody sacrifice of Christ, which without intermission of time is renewed on our altars in an unbloody manner, "For the victim is one and the same, the same now offering by the ministry of priests, who then offered Himself on the cross, the manner alone of offering being different" ( Council of Trent , Session XXIII, Chapter 2). Wherefore with this most august Eucharistic Sacrifice there ought to be joined an oblation both of the ministers and of all the faithful, so that they also may "present themselves living sacrifices, holy, pleasing unto God" ( Romans xii, 1). Nay more, St. Cyprian does not hesitate to affirm that "the Lord's sacrifice is not celebrated with legitimate sanctification, unless our oblation and sacrifice correspond to His passion" ( Ephesians 63). For this reason, the Apostle admonishes us that "bearing about in our body the mortification of Jesus" (2 Corinthians iv, 10), and buried together with Christ, and planted together in the likeness of His death (Cf. Romans vi, 4-5), we must not only crucify our flesh with the vices and concupiscences (Cf. Galatians v, 24), "flying the corruption of that concupiscence which is in the world" (2 Peter i, 4), but "that the life also of Jesus may be made manifest in our bodies" (2 Corinthians iv, 10) and being made partakers of His eternal priesthood we are to offer up "gifts and sacrifices for sins" ( Hebrews v, 1). Nor do those only enjoy a participation in this mystic priesthood and in the office of satisfying and sacrificing, whom our Pontiff Christ Jesus uses as His ministers to offer up the clean oblation to God's Name in every place from the rising of the sun to the going down ( Malachias i, 11), but the whole Christian people rightly called by the Prince of the Apostles "a chosen generation, a kingly priesthood" (1 Peter ii, 9), ought to offer for sins both for itself and for all mankind (Cf. Hebrews v, 3), in much the same manner as every priest and pontiff "taken from among men, is ordained for men in the things that appertain to God" ( Hebrews v, 1).
Source: Miserentissimus Redemptor (Vatican.va)