Gregory the Great — as excerpted in the Catena Aurea on Luke 15:1-7

“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.”