4 After this, to obtain yet more acceptance for His saying, He establishes it not by the honor only, but also by the punishment, going on to say, “And whoso shall offend one of these little ones, it were better for him that a millstone were hanged about his neck, and that he were drowned in the depth of the sea.”
“For as they,” says He, who honor these for my sake, have heaven, or rather an honor greater than the very kingdom; even so they likewise who dishonor them (for this is to offend them), shall suffer the extremity of punishment. And marvel thou not at His calling the affront “an offense;” for many feeble-minded persons have suffered no ordinary offense from being treated with slight and insult. To heighten therefore and aggravate the blame, He states the mischief arising therefrom.
And He does not go on to express the punishment in the same way, but from the things familiar to us, He indicates how intolerable it is. For when He would touch the grosser sort most sharply, He brings sensible images. Wherefore here also, meaning to indicate the greatness of the punishment they shall undergo, and to strike into the arrogance of those that despise them, He brought forward a kind of sensible punishment, that of the millstone, and of the drowning. Yet surely it were suitable to what had gone before to have said, “He that receives not one of these little ones, receives not me;” a thing bitterer than any punishment; but since the very unfeeling, and exceeding gross, were not so much penetrated by this, terrible as it is, He puts “a millstone,” and “a drowning.” And He said not, “A millstone shall be hanged about his neck,” but, “It were better for him” to undergo this; implying that another evil, more grievous than this, awaits him; and if this be unbearable, much more that.
Do you see how in both respects He made His threat terrible, first by the comparison with the known image rendering it more distinct, then by the excess on its side presenting it to the fancy as far greater than that visible one. Do you see how He plucks up by the root the spirit of arrogance; how He heals the ulcer of vainglory; how He instructs us in nothing to set our heart on the first honors; how He persuades such as covet them in everything to follow after the lowest place?
Source: Homilies on the Gospel of St. Matthew (New Advent)