Luke 15:4

Rich then is that Shepherd of whom we all are a hundredth part; and hence it follows, And if he lose one of them, does he not leave &c.

(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.

We may hence understand the extent of our Saviour's kingdom. For He says there are a hundred sheep, bringing to a perfect sum the number of rational creatures subject to Him. For the number hundred is perfect, being composed of ten decades. But out of these one has wandered, namely, the race of man which inhabits earth. But was He then angry with the rest, and moved by kindness only to one? By no means. For they are in safety, the right hand of the Most Mighty being their defence. It behoved Him rather to pity the perishing, that the remaining number might not seem imperfect. For the one being brought back, the hundred regains its own proper form.

(in Hom. 34. in Evang.) 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.