3 “But what is this to the Jews?” one may say, “for they know not even what you say.” “Why, for this cause,” says he, “do I so speak, in order that being roused by the obscurity of my words, they may proceed to seek Him, whom I preach.” In point of fact, he so excited them with good hopes when they came near, that even many publicans and soldiers inquired what they should do, and how they should direct their own life; which was a sign of being thenceforth set free from all worldly things, and of looking to other greater objects, and of foreboding things to come. Yea, for all, both the sights and the words of that time, led them unto lofty thoughts.
Conceive, for example, how great a thing it was to see a man after thirty years coming down from the wilderness, being the son of a chief priest, who had never known the common wants of men, and was on every account venerable, and had Isaiah with him. For he too was present proclaiming him, and saying, “This is he who I said should come crying, and preaching throughout the whole wilderness with a clear voice.” For so great was the earnestness of the prophets touching these things, that not their own Lord only, but him also who was to minister unto Him, they proclaimed a long time beforehand, and they not only mentioned him, but the place too in which he was to abide, and the manner of the doctrine which he had to teach when he came, and the good effect that was produced by him.
See, at least, how both the prophet and the Baptist go upon the same ideas, although not upon the same words.
Thus the prophet says that he shall come saying, “Prepare ye the way of the Lord, make his paths straight.” And he himself when he had come said, “Bring forth fruits meet for repentance,” which corresponds with, “Prepare ye the way of the Lord.” Do you see that both by the words of the prophet, and by his own preaching, this one thing is manifested alone; that he had come, making a way and preparing beforehand, not bestowing the gift, which was the remission, but ordering in good time the souls of such as should receive the God of all?
But Luke expresses somewhat further: not repeating the exordium, and so passing on, but setting down likewise all the prophecy. “For every valley,” says he, “shall be filled; and every mountain and hill shall be brought low; and the crooked shall be made straight, and the rough ways smooth; and all flesh shall see the salvation of God.” Do you perceive how the prophet has anticipated all by his words; the concourse of the people, the change of things for the better, the easiness of that which was preached, the first cause of all that was occurring, even if he has expressed it rather as in figure, it being in truth a prophecy which he was uttering? Thus, when he says, “Every valley shall be filled, and every mountain and hill shall be brought low, and the rough ways shall be made smooth;” he is signifying the exaltation of the lowly, the humiliation of the self-willed, the hardness of the law changed into easiness of faith. For it is no longer toils and labors, says he, but grace, and forgiveness of sins, affording great facility of salvation. Next he states the cause of these things, saying, “All flesh shall see the salvation of God;” no longer Jews and proselytes only, but also all earth and sea, and the whole race of men. Because by “the crooked things” he signified our whole corrupt life, publicans, harlots, robbers, magicians, as many as having been perverted before afterwards walked in the right way: much as He Himself likewise said, “publicans and harlots go into the kingdom of God before you,” because they believed. And in other words also again the prophet declared the self-same thing, thus saying, “Then wolves and lambs shall feed together.” For like as here by the hills and valleys, he meant that incongruities of character are blended into one and the same evenness of self-restraint, so also there, by the characters of the brute animals indicating the different dispositions of men, he again spoke of their being linked in one and the same harmony of godliness. Here also, as before, stating the cause. That cause is, “There shall be He that rises to reign over the Gentiles, in Him shall the Gentiles trust:” much the same as here too he said, “All flesh shall see the salvation of God,” everywhere declaring that the power and knowledge of these our Gospels would be poured out to the ends of the world, converting the human race, from a brutish disposition and a fierce temper to something very gentle and mild.
Source: Homilies on the Gospel of St. Matthew (New Advent)