Then the prophet adds, Rend your hearts and not your garments. By these words he is accusing the ancient people of hardness of heart and meaningless superstition. They frequently rent their garments, but not their hearts. … Let the Spirit rend your heart with his sword, which is the word of God; let him rend it and speedily shatter it into many fragments. There is no way to turn to the Lord with all your heart except your heart be rent. …
Listen to one whom God found to be after his own heart: My heart is ready, O God, my heart is ready, he says – ready for both adversity and prosperity; ready for what is low and what is lofty; ready for whatever you command. …
Let us rend our hearts, beloved, but keep our garments whole. Our garments are our virtues; love is a good garment; obedience is a good garment. Happy is the one who cares for these garments that he may not walk naked. Happy are those whose sins are covered; love covers a multitude of sins. Let us rend our hearts, as was said before, that we may keep our garments whole, as was our Savior’s tunic. The rending of the heart not only keeps the garment whole, but also makes it long and of many colors, like the coat the holy patriarch Jacob gave the son whom he loved more than the others. From this comes perseverance in virtue, from this the many-colored unity of a beautiful way of life. From this comes the glory of the king’s daughter, in golden borders, clothed with many colors.
We can also take the rending of the heart in another way: if the heart is wicked it may be rent by confession; if hard, by compassion. Is not an ulcer rent so that the diseased matter may flow out? Is not the heart rent to overflow in compassion? Both rendings are expedient, that the poison of sin may not be hidden in the heart, and we may not shut off our compassion from our neighbor’s need, that we may receive mercy from Our Lord Jesus Christ, who is over all, blessed forever.
Bernard of Clairvaux, Sermons for Lent and the Easter Season, Lent – Sermon Two: How We Should be Converted to the Lord, v. 5, 6 (CF 52, page 33-35)