Gregory the Great
Catena Aurea · Lukasevangelium 15
Then drew near unto him all the Publicans and sinners for to hear him. And the Pharisees and Scribes murmured, saying, This man receiveth sinners, and eateth with them. And he spake this parable unto them, saying, What man of you, having an hundred sheep, if he lose one of them, doth not leave the ninety and nine in the wilderness, and go after that which is lost, until he find it? And when he hath found it, he layeth it on his shoulders, rejoicing. And when he cometh home, he calleth together his friends and neighbours, saying unto them, Rejoice with me; for I have found my sheep which was lost. I say unto you, that likewise joy shall be in heaven over one sinner that repenteth, more than over ninety and nine just persons, which need no repentance.
(ubi sup.) But he allows there is more joy in heaven over the converted sinner, than over the just who remain stedfast; for the latter for the most part, not feeling themselves oppressed by the weight of their sins, stand indeed in the way of righteousness, but still do not anxiously sigh after the heavenly country, frequently being slow to perform good works, from their confidence in themselves that they have committed no grievous sins. But, on the other hand, sometimes those who remember certain iniquities that they have committed, being pricked to the heart, from their very grief grow inflamed towards the love of God; and because they consider they have wandered from God, make up for their former losses by the succeeding gains. Greater then is the joy in heaven, just as the leader in battle loves that soldier more who having turned from flight, bravely pursues the enemy, than him who never turned his back and never did a brave act. So the husbandman rather loves that land which after bearing thorns yields abundant fruit, than that which never had thorns, and never gave him a plentiful crop. But in the mean time we must be aware that there are very many just men in whose life there is so much joy, that no penitence of sinners however great can in any way be preferred to them. Whence we may gather what great joy it causes to God when the just man humbly mourns, if it produces joy in heaven when the unrighteous by his repentance condemns the evil that he has done.