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

“(in Hom. 34. in Evang.) From which we may gather, that true justice feels compassion, false justice scorn, although the just are wont rightly to repel sinners. But there is one act proceeding from the swelling of pride, another from the zeal for discipline. For the just, though without they spare not rebukes for the sake of discipline, within cherish sweetness from charity. In their own minds they set above themselves those whom they correct, whereby they keep both them under by discipline, and themselves by humility. But, on the contrary, they who from false justice are wont to pride themselves, despise all others, and never in mercy condescend to the weak; and thinking themselves not to be sinners, are so much the worse sinners. Of such were the Pharisees, who condemning our Lord because He received sinners, with parched hearts reviled the very fountain of mercy. But because they were so sick that they knew not of their sickness, to the end that they might know what they were, the heavenly Physician answers them with mild applications. For it follows, And he spake this parable unto them, saying, What man of you having an hundred sheep, and if he lose one of them, does not go after it, &c. He gave a comparison which man might recognise in himself, though it referred to the Creator of men. For since a hundred is a perfect number, He Himself had a hundred sheep, seeing that He possessed the nature of the holy angels and men. Hence he adds, Having an hundred sheep.”