Thus the prophet saith that he shall come saying, “Prepare ye the way of the Lord, make his paths straight.” And he himself when he was come said, “Bring forth fruits meet for repentance,” which corresponds with, “Prepare ye the way of the Lord.” Seest thou that both by the words of the prophet, and by his own preaching, this one thing is manifested alone; that he was 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,” saith 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.”1 Dost thou perceive how the prophet hath 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 hath expressed it rather as in figure, it being in truth a prophecy which he was uttering? Thus, when he saith, “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.
John Chrysostom “Homilies on Mathew”