201 The sacrifices and sufferings required by these acts of love of neighbour unite us to the passion of Christ. In this way, “by that mystic crucifixion of which the Apostle speaks, we shall receive the abundant fruits of its propitiation and expiation, for ourselves and for others”. Christ alone saves us by his offering on the cross; he alone redeems us, for “there is one God; there is also one mediator between God and men, the man Christ Jesus, who gave himself as a ransom for all” (1 Tim 2:5-6). The reparation that we offer is a freely accepted participation in his redeeming love and his one sacrifice. We thus complete in our flesh “what is lacking in Christ’s afflictions for the sake of his body, that is, the Church” ( Col 1:24); and Christ himself prolongs through us the effects of his complete and loving self-oblation.
Source: Dilexit Nos (Vatican.va)