Thus the meaning of the saying is this. If a Christian sincerely recognizes his sins before God, is contrite over them, and asks God’s forgiveness for them for the sake of the merits of Christ the Savior on the Cross, and if he confesses them before his spiritual father, while nourishing the desire and intention to avoid sins, then he will receive their forgiveness from God. The mystery of repentance is thus the necessary condition of the Christian’s reconciliation with God by the sacrifice of Christ on the Cross.
“The apostle repeats his discourse several times, so that through abundant and frequent rebukes he may show them the whole greatness of the transgression and incline them to confession. How great a good is born from confession is clear from the following words: say your sins first, that you may be justified (Is 43:26). It is customary for a teacher to repeat the same thing often for a beginning disciple, first briefly, then more fully, in order to impart clearer knowledge” (Theophylact).
“Faithful and righteous.” God manifests His faithfulness in His promises and in fulfilling them (cf. Heb 10:23; Heb 11:1; 1 Cor 1:9; 10:13; 1 Thess 5:24). What He promises to do, He will do, because He is always faithful to His word, as true in all His words and deeds. “And righteous” means just, rendering to each and to all according to deeds, fair and impartial.
Or, as blessed Theophylact interprets it: “God is faithful. This is the same as true. For the word faithful is used not only of one to whom something is entrusted, but also of one who is himself most faithful and who, by his own faithfulness, can make others such. In this sense God is faithful. He is righteous in the sense that He does not drive away those who come to Him, however sinful they may be” (see Jn 6:37).
Applied to the forgiveness of human sins and the cleansing of their unrighteousness, the expression has this meaning. However heavy our sins may be, we must not be troubled or despair, fearing God’s righteous judgment and wrath. He will forgive us our transgressions, because He cannot be unfaithful to His known promises concerning the reception of sinners being saved. For by Christ’s blood the justice of God is fully satisfied, and by His sacrifice every sin is forgiven to man and every unrighteousness is cleansed.
“He will forgive,” or leave. He will not only remove the guilt of sins as Judge, but will destroy or wipe out the very sins, so that no remembrance of them will remain either in thought or in deed. He will cleanse, or wash, by the blood of Christ, and destroy impurity. The cleansing of sins and the removal of every impurity are the fruit of the redemptive sacrifice of Christ the Savior. Our confession of sins and contrition are the conditions on our side, so that we may be able to appropriate the saving action of the Redeemer’s merits.