Augustine of Hippo
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.
(de Quæst. Ev. lib. 2. qu. 32.) Or He spoke of those ninety and nine whom He left in the wilderness, signifying the proud, who bear solitude as it were in their mind, in that they wish to appear themselves alone, to whom unity is wanting for perfection. For when a man is torn from unity, it is by pride; since desiring to be his own master, he follows not that One which is God, but to that One God ordains all who are reconciled by repentance, which is obtained by humility.