The washing away of sins is indicated by the white robes of the catechumens, whence the Church speaks of herself as black and comely. Angels marvel at her brightness as at that of the flesh of the Lord. Moreover, Christ Himself commended His beauty to His Spouse under many figures. The mutual affection of the one for the other is described.
34. After this white robes were given to you as a sign that you were putting off the covering of sins, and putting on the chaste veil of innocence, of which the prophet said: “You shall sprinkle me with hyssop and I shall be cleansed, You shall wash me and I shall be made whiter than snow.” For he who is baptized is seen to be purified both according to the Law and according to the Gospel: according to the Law, because Moses sprinkled the blood of the lamb with a bunch of hyssop; according to the Gospel, because Christ's garments were white as snow, when in the Gospel He showed forth the glory of His Resurrection. He, then, whose guilt is remitted is made whiter than snow. So that God said by Isaiah: “Though your sins be as scarlet, I will make them white as snow.”
35. The Church, having put on these garments through the laver of regeneration, says in the Song of Songs: “I am black and comely, O daughters of Jerusalem.” Black through the frailty of her human condition, comely through the sacrament of faith. And the daughters of Jerusalem beholding these garments say in amazement: “Who is this that comes up made white?” She was black, how is she now suddenly made white?
36. The angels, too, were in doubt when Christ arose; the powers of heaven were in doubt when they saw that flesh was ascending into heaven. Then they said: “Who is this King of glory?” And while some said “Lift up your gates, O princes, and be lifted up, you everlasting doors, and the King of glory shall come in.” In Isaiah, too, we find that the powers of heaven doubted and said: “Who is this that comes up from Edom, the redness of His garments is from Bosor, He who is glorious in white apparel?”
37. But Christ, beholding His Church, for whom He Himself, as you find in the book of the prophet Zechariah, had put on filthy garments, now clothed in white raiment, seeing, that is, a soul pure and washed in the laver of regeneration, says: “Behold, you are fair, My love, behold you are fair, your eyes are like a dove's,” in the likeness of which the Holy Spirit descended from heaven. The eyes are beautiful like those of a dove, because in the likeness of a dove the Holy Spirit descended from heaven.
38. And farther on: “Your teeth are like a flock of sheep that are shorn, which have come up from the pool, which all bear twins, and none is barren among them, your lips are as a cord of scarlet.” This is no slight praise. First by the pleasing comparison to those that are shorn; for we know that goats both feed in high places without risk, and securely find their food in rugged places, and then when shorn are freed from what is superfluous. The Church is likened to a flock of these, having in itself the many virtues of those souls which through the laver lay aside the superfluity of sins, and offer to Christ the mystic faith and the grace of good living, which speak of the cross of the Lord Jesus.
39. The Church is beautiful in them. So that God the Word says to her: “You are all fair, My love, and there is no blemish in you,” for guilt has been washed away. “Come hither from Lebanon, My spouse, come hither from Lebanon, from the beginning of faith will you pass through and pass on,” because, renouncing the world, she passed through things temporal and passed on to Christ. And again, God the Word says to her: “How beautiful and sweet are you made, O love, in your delights! Your stature has become like that of a palm-tree, and your breasts like bunches of grapes.”
40. And the Church answers Him, “Who will give You to me, my Brother, that sucked the breasts of my mother? If I find You without, I will kiss You, and indeed they will not despise me. I will take You, and bring You into the house of my mother; and into the secret chamber of her that conceived me. You shall teach me.” You see how, delighted with the gifts of grace, she longs to attain to the innermost mysteries, and to consecrate all her affections to Christ. She still seeks, she still stirs up His love, and asks of the daughters of Jerusalem to stir it up for her, and desires that by their beauty, which is that of faithful souls, her spouse may be incited to ever richer love for her.
41. So that the Lord Jesus Himself, invited by such eager love and by the beauty of comeliness and grace, since now no offenses pollute the baptized, says to the Church: “Place Me as a seal upon your heart, as a signet upon your arm;” that is, you are comely, My beloved, you are all fair, nothing is wanting to you. Place Me as a seal upon your heart, that your faith may shine forth in the fullness of the sacrament. Let your works also shine and set forth the image of God in the Whose image you were made. Let no persecution lessen your love, which many waters cannot quench, nor many rivers drown.
42. And then remember that you received the seal of the Spirit; the spirit of wisdom and understanding, the spirit of counsel and strength, the spirit of knowledge and godliness, and the spirit of holy fear, and preserved what you received. God the Father sealed you, Christ the Lord strengthened you, and gave the earnest of the Spirit in your heart, as you have learned in the lesson from the Apostle.
Source: On the Mysteries (New Advent)