Ephrem the Syrian : 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 John Cassian : 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, and my burden is light." The way of the Lord then has refreshment if it is kept according to his law. — CONFERENCE 24:25 Theodoret of Cyrus : He has clearly shown by these terms the facility of the evangelical proclamation: thanks to this facility, in a short time it filled the whole world. One should understand by "filled valleys" and "mountains and hills brought low" that literally it relates to a way that is level and cleared of all obstacles, but in a figurative sense, the souls of the unbelievers are the valleys, those who are lying in some way at the bottom of a pit and who do not have the intelligence to turn toward the heavens, while the mountains and the hills are the demons who are worshiped on the mountains and on the hills, whose deceit has been ended by the incarnation of our Savior. — COMMENTARY ON Isaiah 12:40.4