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.
One sheep then perished when man by sinning left the pastures of life. But in the wilderness the ninety and nine remained, because the number of the rational creatures, that is to say of Angels and men who were formed to see God, was lessened when man perished; and hence it follows, Does he not leave the ninety and nine in the wilderness, because in truth he left the companies of the Angels in heaven. But man then forsook heaven when he sinned. And that the whole body of the sheep might be perfectly made up again in heaven, the lost man was sought for on earth; as it follows, And go after that &c.