Every valley shall be exalted, and every mountain and hill shall be made low: and the crooked shall be made straight, and the rough places smooth:
Read Chapter 40
Aquinas Study Bible
The true consolation, the genuine comfort and the real deliverance from the iniquities of men is the incarnation of our God and Savior. Now the first who acted as herald of this event was the inspired John the Baptist. Accordingly, the prophetic text proclaims the realities that relate to him in advance: for that is what the three blessed Gospel writers have taught us and that the most divine Mark has even made the prologue of his work. (Theodoret of Cyrus) Orderly is the arrangement of the teachings of God; in Isaiah: make right his path. [Is. 40:3] Crooked is deviance from the teachings of God; in Isaiah: and they were crooked in order and rough in the level ways. [Is. 40:4] Obedience to the teachings of God is smooth; in Isaiah: the same as above. Disobedience is, in contrast, harsh and bristly; in Isaiah: the same as above. (Eucharius of Lyons Formulae Bk 8)
It is said that the valleys are filled, because the Lord has entered the world, and has redeemed all the peoples from the bondage of the devil, and brought them back to the faith and adoration of their Creator and has taught them to hope in eternal salvation. This is also said, because he has filled the deep valleys and the horrible and inhospitable caves with ascetics, who, after abandoning the cares of this world, exclusively devote themselves to honor and praise God. - "Commentary on Isaiah 40.4"
According to the plain teaching of the Lord, the king’s highway is easy and smooth, though it may be felt as hard and rough. For those who piously and faithfully serve him, when they have taken on them the yoke of the Lord and have learned of him, that he is meek and lowly of heart, at once [they] somehow or other lay aside the burden of earthly passions and find no labor but rest for their souls, by the gift of the Lord. [To this] he himself testifies by Jeremiah the prophet, saying, “Stand on the ways and see, and ask for the old paths, which is the good way, and walk in it: and you shall find refreshment for your souls.” For to them at once “the crooked shall become straight and the rough ways plain”; and they shall “taste and see that the Lord is gracious.” And when they hear Christ proclaiming in the Gospel, “Come unto me, all you that labor and are heavy laden, and I will refresh you,” they will lay aside the burden of their sins and realize what follows: “For my yoke is easy, an...