5 And when he hath found it, lay it upon his shoulders, rejoicing:
Gregory of Nyssa: He did not punish it, he did not get it to the flock by driving it, but by placing it upon his shoulder, and carrying it gently, he united it to his flock.
Gregory: He placed the sheep upon his shoulders, for taking man’s nature upon Him he bore our sins.
6 And coming home, call together his friends and neighbours, saying to them: Rejoice with me, because I have found my sheep that was lost.
Gregory: Our Shepherd having restored man, returns to his heavenly kingdom. . . . By his friends and neighbors He means the companies of Angels, who are His friends because they are keeping His will in their own steadfastness; they are also His neighbors, because by their own constant waiting upon Him they enjoy the brightness of His sight.
Theophylact: The heavenly powers thus are called sheep, because every created nature as compared with God is as the beasts, but inasmuch as it is rational, they are called friends and neighbors.
Gregory: He says not, “Rejoice with the sheep that is found,” but with me, because truly our life is His joy, and when we are brought home to heaven we fill up the festivity of His joy.
7 I say to you, that even so there shall be joy in heaven upon one sinner that doth penance, more than upon ninety-nine just who need not penance.
Ambrose: The angels, inasmuch as they are intelligent beings, do not unreasonably rejoice at the redemption of men. . . . Let this serve as an incentive to goodness, for a man to believe that his conversion will be pleasing to the assembled angels, whose favor he ought to court, or whose displeasure to fear.
Gregory: There is more joy in heaven over the converted sinner, than over the just who remain steadfast; for the latter for the most part, not feeling themselves oppressed by the weight of their sins, stand indeed in the way of righteousness, but still do not anxiously sigh after the heavenly country, frequently being slow to perform good works, from their confidence in themselves that they have committed no grievous sins. But, on the other hand, sometimes those who remember certain iniquities that they have committed, being pricked to the heart, from their very grief grow inflamed towards the love of God; and because they consider they have wandered from God, make up for their former losses by the succeeding gains. Greater then is the joy in heaven, just as the leader in battle loves that soldier more who having turned from flight, bravely pursues the enemy, than him who never turned his back and never did a brave act. So the husbandman rather loves that land which after bearing thorns yields abundant fruit, than that which never had thorns, and never gave him a plentiful crop.
Scripture from the Douay-Rheims Bible. Commentary from St. Thomas Aquinas, Catena Aurea, Vol. III (London: Rivington, 1843).