Catena Aurea on Luke 15:1–7: Augustine of Hippo

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.