Ambrose of Milan
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.
Now the angels, inasmuch as they are intelligent beings, do not unreasonably rejoice at the redemption of men, as it follows, I say unto you, that likewise joy shall be in heaven over one sinner that repenteth, more than over ninety and nine just persons who need no repentance. Let this serve as an incentive to goodness, for a man to believe that his conversion will be pleasing to the assembled angels, whose favour he ought to court, or whose displeasure to fear.