See the notes at Matthew 18:12-13.

What man of you - Our Lord spoke this and the following parable to justify his conduct in receiving and conversing with sinners or heathens.
A hundred sheep - Parables similar to this are frequent among the Jewish writers. The whole flock of mankind, both Jews and Gentiles, belongs unto this Divine Shepherd; and it is but reasonable to expect, that the gracious proprietor will look after those who have gone astray, and bring them back to the flock. The lost sheep is an emblem of a heedless, thoughtless sinner: one who follows the corrupt dictates of his own heart, without ever reflecting upon his conduct, or considering what will be the issue of his unholy course of life. No creature strays more easily than a sheep; none is more heedless; and none so incapable of finding its way back to the flock, when once gone astray: it will bleat for the flock, and still run on in an opposite direction to the place where the flock is: this I have often noticed. No creature is more defenceless than a sheep, and more exposed to be devoured by dogs and wild beasts. Even the fowls of the air seek their destruction. I have known ravens often attempt to destroy lambs by picking out their eyes, in which, when they have succeeded, as the creature does not see whither it is going, it soon falls an easy prey to its destroyer. Satan is ever going about as a roaring lion seeking whom he may devour; in order to succeed, he blinds the understanding of sinners, and then finds it an easy matter to tumble them into the pit of perdition. Who but a Pharisee or a devil would find fault with the shepherd who endeavors to rescue his sheep from so much danger and ruin!

What man of you having an hundred sheep,.... A flock of sheep, consisting of such a number; See Gill on Matthew 18:12,
if he lose one of them, by straying from the flock,
doth not leave the ninety and nine in the wilderness, upon the common where they were feeding,
and go after that which is lost until he find it? by which parable Christ vindicates his conduct in conversing with sinners, and neglecting the Scribes and Pharisees; for if it was right for an owner of an hundred sheep, when he had lost one of them, to leave all the rest, and go in search after that one till he had found it; then it was right in Christ to do what he did. The Jewish nation seems to be designed "by the hundred sheep", who are frequently represented as a flock of sheep, Psalm 77:20 which are divided into ninety nine, and one: for by the "ninety nine" left in the wilderness, cannot be meant angels, as some have thought; for angels are never called sheep; and besides, the one lost sheep is of the same kind with the ninety and nine; and, according to this sense, must design an angel, or angels likewise; whereas none of the fallen angels are sought up, recovered, and saved. Moreover, when Christ became incarnate, he did not leave the angels; they accompanied and attended him in his state of humiliation; and much less in a wilderness, and still less can heaven be so called; to which may be added, that the angels in heaven are distinguished from the ninety nine as well as from the one lost sheep in Luke 15:7 nor can elect men be designed by them, who are already called by grace, whether they be in heaven or on earth; for though they in heaven are the spirits of just men made perfect, and are in a state that need no repentance, yet it cannot be said of them, that they went not astray, as in Matthew 18:13 for all God's people have been like sheep going astray, and were as such considered when Christ was here on earth, and bore their sins; and especially those could never be said to be left in a wilderness: nor the saints on earth: for though they are just persons, being justified by the righteousness of Christ, yet they daily need repentance; nor can it be said of them that they went not astray; nor are they left by Christ in the wilderness of this world; nor can there be more joy in heaven over one repenting sinner, than over these; but the self-righteous Scribes and Pharisees, that murmured at our Lord's receiving sinners, are meant. These were sheep, at least were in sheep's clothing; they were nominal professors, and belonged to the Jewish fold, or national church state; their number was ninety nine, to one; which is not to be taken strictly, as though only one in a hundred of them were saved; but it shows, that the greater part of the Jews were of this sort. The dividing of an hundred after this manner, into ninety nine and one, was usual with the Jews; so in their traditions (p), concerning distributing filberts to the poor,
"R. Simeon says, if "ninety nine" say "divide", and "one" says spoil, or scatter, they hearken to him, because he speaks according to the constitution; but of a vine and date, it is not so: if "ninety and nine" say spoil, and "one" says divide, they hearken to him, for he speaks according to the constitution.''
And elsewhere (q) they say,
""ninety and nine" die by an (evil) eye, and "one" by the hand of heaven; R. Chanina and Samuel, both of them say, "ninety and nine" die by cold, and "one" by the hand of heaven---R. Samuel bar Nachman, in the name of R. Jonathan says, "ninety and nine" die by heat, and "one" by the hand of heaven; and the Rabbans say, "ninety and nine" die by transgression, and "one" by the hand of heaven. Says R. Eleazar, "ninety and nine" die by bitterness, and "one" by the hand of heaven.''
And in another place (r) it is said,
""ninety and nine" die by an evil eye, and "one" by the way of the earth;''
in the common way: once more it is said (s),
"of the "hundred" cries which a woman cries, when she sits upon the stool (in travail), "ninety and nine" are death, and "one" for life.''
And this way of speaking also prevailed in other eastern nations, as in Arabia; in the Alcoran of Mahomet (t) there is such an expression as this;
"this my brother had "ninety nine sheep", and I had only "one" ewe.''
The "one lost sheep" in this parable, though it may include all the elect of God, and be accommodated to a single elect sinner, yet chiefly respects the chosen of God among the Jews; which were very few, a remnant according to the election of grace: and which lay among the profane part of them, the publicans and sinners; Who are particularly pointed out here, as appears from the context: these are called "sheep", even before conversion; not because they had the agreeable properties of sheep, for they were all the reverse; nor could some things be said of them before as after, as, that they heard the voice of Christ, and followed him; nor because they were unprejudiced against, and predisposed to receive the Gospel: but they are so called by anticipation, because they would be so; or rather in virtue of electing grace, by which they were chosen, and separated from others, and made the care and charge of Christ the great shepherd, and were the sheep of his hand: these are represented as going astray from the shepherd, and from the fold, and out of the right way; and who being like sheep, stupid and insensible of their danger, wander about, and never return of themselves till they are returned to, and by the great shepherd and bishop of souls. And in their unregenerate estate they are lost sheep, not irretrievably and eternally lost, as the world's goats; for though they are lost in Adam, yet not in Christ; and though lost in themselves, so as there is no possibility of ever recovering and saving themselves; yet as they were preserved in Christ, they are recovered and saved by him; who is the owner and proprietor of the whole flock, of all the "hundred" sheep, of the whole body of the Jewish nation; who were his by creation, and by being chosen from, and above all other people; and were distinguished by peculiar favours, had the "Shekinah", and presence of God among them, and his worship, word, and ordinances. Christ was peculiarly promised to them, and was born of them; and was a minister of the circumcision, being sent only to the lost sheep of the house of Israel: though the "ninety and nine" were not his sheep in the most peculiar sense, or in such sense as the "one" lost sheep, which were his by his Father's gift, as all the elect are; hence he knows them, calls them, and receives them, and keeps them, and highly values them: he had them, they were put into his hands, he took the care and charge of there, he undertook to bring them in, to feed them, to die for them, and save them; and they are his by purchase, and he asserts his right to them, by calling them by his grace, and will distinguish them as his own, at the last day: and now, because of the different interest Christ has in the ninety and nine, and the one, different regards are had to them; the ninety and nine, the self-righteous Scribes and Pharisees,
he leaves in the wilderness, in a state of unregeneracy; so called, because in those that are in such a state, nothing is sown or planted, what grows there is natural; there is no seed of grace, no plants of pleasure, no ingrafted word, no fruits of righteousness, nothing but thorns and briers, of sins and corruptions: and also because of the want of provisions; no bread of life, nor water of life; no sincere milk of the word, no breasts of consolation; nothing but husks, and bread of deceit: and it is like a wilderness, because of solitariness; such as are in this state, are alone, without God, and Christ, and the blessed Spirit; they are aliens from the people of God, and converse with none but wicked men, comparable to savage beasts of the desert: moreover, it may be so called, because of the various perplexing cross ways in it; the ways of sin are many, and crooked, and dark; and indeed, such are the religious ways of unregenerate men: to which may be added, that it bears this name, because of the danger of it; for such as are in it are exposed to beasts of prey; particularly to Satan, the red dragon, and roaring lion; and to pits, gins, and snares, to hellfire, destruction, and misery. Christ's "leaving" persons in such a state, supposes they were in it, antecedent to their being left: man was originally placed in a garden, sinning against God, he forfeited his happy situation, and was drove out from it; and wandering from God he fell into this wilderness state. Christ does not lead any into it, but leaves them in it; which is done in consequence of his Father's act of preterition, or passing them by when he chose others; and this he does, when he does not call them by his grace, as he does others; does not manure, till, and cultivate them as he does his own husbandry and vines; makes no provision of food and pasture for them; leaves them to themselves, and without the enjoyment of himself; to follow their own ways, without a guide, and to beasts of prey without a guard. Now the persons he thus leaves are such whom the Father has left out in his choice and covenant; and who left Christ, rejected and despised him; and were persons that made great pretensions to religion, were righteous in their own eyes, and in their own account never were astray, nor needed repentance. On the other hand, the one lost sheep, the chosen of God among publicans and sinners, a special regard is had to them: Christ
goes after that which is lost until he find it; not after all mankind; for though they are all lost, yet they are not all redeemed by Christ; nor are they made sensible of their lost condition; nor effectually called by grace; nor brought home: nor does he go after the ninety nine, for Christ came not to call the righteous; though these were lost, and irretrievably too, yet they were not sensible of their condition: but God's elect among the Jews are the persons here said to be lost; to show their common condition with the rest of mankind; to express the love of Christ towards them the more; and to magnify the riches of his grace in their salvation: these he went after in redemption, he came forth from his Father, and came down from heaven for their sakes; he died to gather them together, and represented them all in his sufferings and death; he bore all their sins, and made reconciliation for them, and procured the full pardon of them; he satisfied the law and justice of God, wrought out an everlasting righteousness, and obtained eternal redemption, and a complete salvation for them: and he went after them in the effectual calling; before conversion an elect sinner is without Christ, and goes astray from him; nor does he ever come to Christ till Christ comes after him, and lays hold upon him; he sends his ministers after such, and his Spirit into them, and comes himself, and takes possession of them. To find his lost sheep by redeeming grace he came into this world, a world of wickedness, sorrow, and trouble, of cruelty, and barbarity; and the reason of his coming here was, because his sheep were here; he came after them, and on their account: and to find them by effectual calling, he still comes into the world by his word and Spirit; God's elect are in the world, Christ sends his Gospel into it, and by his Spirit and grace comes and separates them from the men of it. In Matthew 18:12 he is said to go "into the mountains" after his lost sheep; which, with respect to redemption, may denote the difficulties that lay in the way of it; such as his incarnation, obedience, sufferings and death, and the many enemies he had to grapple with and subdue; and with respect to calling grace, may express the state and condition God's elect are in by nature, being on the mountains of sin, of Sinai, of the law, and of their own righteousness. Now Christ goes after them "till he finds" them; which denotes continuance, his indefatigable industry and diligence, his resolution and courage, and his success. The reasons why he thus goes after them are not their number, for they are the fewest of all; nor their nature, which is no better than others, nor any previous dispositions, or good characters, for those designed here were publicans and sinners; nor any future improvements and service by them, for they were the base and foolish things of this world; nor because near at hand, and so easily looked up, for they were afar off; but because of his love to them, and the relation between them as shepherd and sheep; and because of his Father's will, and his own obligation by covenant; and because of his own interest and glory.
(p) Misn. Pesach. c. 4. sect. 1, 2. (q) T. Hieros. Sabbat, fol. 14. 3. Vajikra Rabba, sect. 16. fol. 158. 4. (r) T. Bab. Bava Metzia, fol. 107. 2. (s) Vajikra Rabba, sect. 27. fol. 168. 3. (t) C. 38.

leave the ninety and nine--bend all His attention and care, as it were, to the one object of recovering the lost sheep; not saying. "It is but one; let it go; enough remain."
go after . . . until, &c.--pointing to all the diversified means which God sets in operation for recovering sinners.

Leave the ninety and nine in the wilderness - Where they used to feed: all uncultivated ground, like our commons, was by the Jews termed wilderness or desert. And go after - In recovering a lost soul, God as it were labours. May we not learn hence, that to let them alone who are in sin, is both unchristian and inhuman! Matthew 18:12.