2 And Jesus called a little child unto him, and set him in the midst of them,
[AD 253] Origen of Alexandria on Matthew 18:1-6
Herein we ought to be imitators of the disciples, that when any question of doubt arises among us, and we find not how to settle it, We should with one consent go to Jesus, Who is able to enlighten the hearts of men to the explication of every perplexity. We shall also consult some of the doctors, who are thought most eminent in the Churches. But in that they asked this question, the disciples knew that there was not an equality among the saints in the kingdom of heaven; what they yet sought to learn was, how they were so, and lived as greater and less. Or, from what the Lord had said above, they knew Who was the best and who was great; but out of many great, who was the greatest, this Was not clear to them.

But how can he who has been converted, and become as a little child, be yet liable to be scandalized? This may be thus explained. Every one who believes on the Son of God, and walks after evangelic acts, is converted and walks as a little child; but he who is not converted that he may become as a child, it is impossible that he should enter into the kingdom of heaven. But in every congregation of believers, there are some only newly converted that they may become as little children, but not yet made such; these are the little ones in Christ, and these are they that receive offence.

[AD 367] Hilary of Poitiers on Matthew 18:1-6
He calls infants all who believe through the hearing of faith; for such follow their father, love their mother, know not to will that which is evil, do not bear hate, or speak lies, trust what is told them, and believe what they hear to be true. But the letter is thus interpreted.

Mystically; The work of the mill is a toil of blindness, for the beasts having their eyes closed are driven round in a circle, and under the type of an ass we often find the Gentiles figured, who are held in the ignorance of blind labour; while the Jews have the path of knowledge set before them in the Law, who if they offend Christ's Apostles it were better for them, that having their necks made fast to a mill-stone, they should be drowned in the sea, that is, kept under labour and in the depths of ignorance, as the Gentiles; for it were better for them that they should have never known Christ, than not to have received the Lord of the Prophets.

[AD 407] John Chrysostom on Matthew 18:1-6
Thus they suffered a human passion, which the Evangelist denotes by saying, At the same time come the disciples to Jesus, saying, Who, we pray thee, is the greatest in the kingdom of heaven? Ashamed to show the feeling which was working within, they do not say openly, Why have you honoured Peter above us? but they ask in general, Who is the greatest? When in the transfiguration they saw three distinguished, namely, Peter, James, and John, they had no such feeling, but now that one is singled out for especial honour, then they are grieved. But do you remember, first, that it was nothing in this world that they sought; and, secondly, that they afterwards laid aside this feeling? Even their failings are above us, whose enquiry is not, Who is the greatest in the kingdom of heaven? but, Who is greatest in the kingdom of the world?

He chose, I suppose, quite an infant, devoid of any of the passions.

Not only if ye become such yourselves, but also if for My sake you shall pay honour to other such, ye receive reward; and as the return for the honour you pay them, I entail upon you the kingdom. He puts indeed what is far greater, Receiveth me.

And to make this word the rather received. He subjoins a penalty in what follows, Whoso offendeth one of these little ones, & c. as though he had said, As those who for My sake honour one of these, have their reward, so they who dishonour shall undergo the extreme punishment. And marvel not that He calls an evil word an offence, for many of feeble spirit are offended by only being despised,

To correspond with the foregoing, He should have said here, Receiveth not Me, which were bitterer than any punishment; but because they were dull, and the before-named punishment did not move them, by a familiar instance He shows that punishment awaited them; for He therefore says, it were better for him, because another more grievous punishment awaits him.

[AD 420] Jerome on Matthew 18:1-6
The disciples seeing one piece of money paid both for Peter and the Lord, conceived from this equality of ransom that Peter was preferred before all the rest of the Apostles.

Jesus seeing their thoughts would heal their ambitious strivings, by arousing an emulation in lowliness; whence it follows, And Jesus calling a little child, set him in the midst of them.

One whose tender age should express to them the innocence which they should have. But truly He set Himself in the midst of them, a little one who had come not to be ministered unto, but to minister; (Mat. 20:28.) that He might be a pattern of holiness. Others interpret the little one of the Holy Spirit, whom He set in the hearts of His disciples, to change their pride into humility. (Vid. Origen. in loc.) And he said. Verily I say unto you, Except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven. He does not enjoin on the Apostles the age, but the innocence of infants, which they have by virtue of their years, but to which these might attain by striving; that they should be children in malice, not in understanding. As though He had said, As this child, whom I set before you as a pattern, is not obstinate in anger, when injured does not bear it in mind, has no emotion at the sight of a fair woman, does not think one thing while he speaks another; so ye, unless ye have the like innocence and purity of mind, shall not be able to enter into the kingdom of heaven.

Or otherwise; Whoso shall humble himself as this little child, that is, whoso shall humble himself after My example, he shall enter into the kingdom of heaven. It follows, And whoso receiveth one such little one in my name, receiveth me.

For whoever is such that he imitates Christ's humility and innocence, Christ is received by him; and by way of caution, that the Apostles should not think, when such are come to them, that it is to themselves that the honour is paid, He adds, that they are to be received not for their own desert, but in honour of their Master.

Observe that he who is offended is a little one, for the greater hearts do not take offences. And though it may be a general declaration against all who scandalize any, yet from the connection of the discourse it may be said specially to the Apostles; for in asking who should be greatest in the kingdom of heaven, they seemed to be contending for preeminence among themselves; and if they had persisted in this fault, they might have scandalized those whom they called to the faith, seeing the Apostles contending among themselves for the preference.

When it is said, It is better for him that a mill-stone be hanged about his neck, He speaks according to the custom of the province; for among the Jews this was the punishment of the greater criminals, to drown them by a stone tied to them. It is better for him, because it is far better to receive a brief punishment for a fault, than to be reserved for eternal torments.

[AD 430] Augustine of Hippo on Matthew 18:1-6
(Quæst. Ev. i. 24) Otherwise; Whoso offendeth one of these little ones, that is so humble as He would have his disciples to be, by not obeying, or by opposing, (as the Apostle says of Alexander,) it were better for him, that a millstone should be hanged about his neck, and he be drowned in the depths of the sea, (2 Tim. 4:15.) that is, it were better for him that desire of the things of the world, to which the blind and foolish are tied down, should sink him by its load to destruction.

[AD 533] Remigius of Rheims on Matthew 18:1-6
In the understanding of grace, or in ecclesiastical dignity, or at least in everlasting blessedness.

[AD 604] Gregory the Dialogist on Matthew 18:1-6
(Mor. vi. 37.) Otherwise; What is denoted by the sea, but the world, and what by the mill-stone, but earthly action? which, when it binds the neck in the yoke of vain desires, sends it to a dull round of toil. There arc some who leave earthly action, and bond themselves to aims of contemplation beyond the reach of intellect, laying aside humility, and so not only throw themselves into error, but also cast many weak ones out of the bosom of truth. Whoso then offends one of the least of mine, it were better for him that a mill-stone be tied about his neck, and he be cast into the sea; that is, it were better for a perverted heart to be entirely occupied with worldly business, than to be at leisure for contemplative studies to the hurt of many.

[AD 1274] Glossa Ordinaria on Matthew 18:1-6
(interlin.) Except ye be converted from this ambition and jealousy in which you are at present, and become all of you as innocent and humble in disposition as you are weak, in your years, ye shall not enter into the kingdom of heaven; and since there is none other road to enter in, whoso shall humble himself as this little child, the same is greatest in the kingdom of heaven; for by how much a man is humble now, by so much shall he be exalted in the kingdom of heaven.

[AD 407] John Chrysostom on Matthew 18:2
And the child which He set in the midst I suppose to have been a very young child indeed, free from all these passions. For such a little child is free from pride and the mad desire of glory, and envy, and contentiousness, and all such passions, and having many virtues, simplicity, humility, unworldliness, prides itself upon none of them; which is a twofold severity of goodness; to have these things, and not to be puffed up about them.
[AD 420] Jerome on Matthew 18:2
He called a child to him to ask its age or to show the image of innocence. Or perhaps he actually set a child in their midst—he himself, who had not come to be served but to serve—to show them an example of humility.

[AD 420] Jerome on Matthew 18:2
(Verse 2.) And calling a little child, he set him in the midst of them, and said. Either simply any little child, to inquire about his age, and to demonstrate the likeness of innocence. Or certainly he placed a little child in their midst, who had come not to be served, but to serve, in order to give them an example of humility. Others interpret the little child as the Holy Spirit, whom he placed in the hearts of the disciples, in order to transform pride into humility.

[AD 1963] CS Lewis on Matthew 18:2
It is, of course, the essence of Christianity that God loves man and for his sake became man and died. But that does not prove that man is the sole end of nature. In the parable, it was the one lost sheep that the shepherd went in search of:” it was not the only sheep in the flock, and we are not told that it was the most valuable — save in so far as the most desperately in need has, while the need lasts, a peculiar value in the eyes of Love. The doctrine of the Incarnation would conflict with what we know of this vast universe only if we knew also that there were other rational species in it who had, like us, fallen, and who needed redemption in the same mode, and that they had not been vouchsafed it. But we know none of these things. It may be full of life that needs no redemption. It may be full of life that has been redeemed. It may be full of things quite other than life which satisfy the Divine Wisdom in fashions one cannot conceive. We are in no position to draw up maps of God’s psychology, and prescribe limits to His interests. We would not do so even for a man whom we knew to be greater than ourselves. The doctrines that God is love and that He delights in men, are positive doctrines, not limiting doctrines. He is not less than this. What more He may be, we do not know; we know only that He must be more than we can conceive.