Matthew 18:2

What then saith Christ? He unveils their conscience, and replies to their feeling, not merely to their words. "For He called a little child unto Him," saith the Scripture, "and said, Except ye be converted, and become as this little child, ye shall not enter into the kingdom of heaven." "Why, you," He saith, "inquire who is greatest, and are contentious for first honors; but I pronounce him, that is not become lowest of all, unworthy so much as to enter in thither." And full well doth He both allege that pattern, and not allege it only, but also set the child in the midst, by the very sight abashing them, and persuading them to be in like manner lowly and artless. Since both from envy the little child is pure, and from vainglory, and from longing for the first place; and he is possessed of the greatest of virtues, simplicity, and whatever is artless and lowly. Not courage then only is wanted, nor wisdom, but this virtue also, humility I mean, and simplicity. Yea, and the things that belong to our salvation halt even in the chiefest point, if these be not with us. The little child, whether it be insulted and beaten, or honored and glorified, neither by the one is it moved to impatience or envy, nor by the other lifted up. Seest thou how again He calls us on to all natural excellencies, indicating that of free choice it is possible to attain them, and so silences the wicked frenzy of the Manichaeans? For if nature be an evil thing, wherefore doth He draw from hence His patterns of severe goodness? And the child which He set in the midst 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.

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.