78 Let us, then, not rejoice over the sin of any one, but rather let us mourn, for it is written: “Rejoice not against me, O my enemy, because I have fallen, for I shall arise; for if I sit in darkness the Lord shall be a light unto me, I will bear the indignation of the Lord, because I have sinned against Him, until He maintain my cause, and execute judgment for me, and bring me forth to the light, and I shall behold His righteousness. Mine enemy, too, shall see it and shall be covered with confusion, which said unto me, Where is the Lord your God? My eyes shall behold her, and she shall be for treading down as the mire in the streets.” And this not unreservedly, for he who rejoices at the fall of another rejoices at the victory of the devil. Let us, then, rather mourn when we hear that one has perished for whom Christ died, Who despises not even the straw in time of harvest.
79. O that He may not cast away this straw at His harvest, the empty stalks of my produce; but may He gather it in, as is said by some one: “Woe is me, for I have become as one that gathers straw in harvest, and grape gleanings in the vintage,” that He may eat of the firstfruits at least of His grace in me, though He approve not the later fruit.
Source: On Repentance (New Advent)