The interpretation timeline
Luke 3:5 Douay-Rheims

Every valley shall be filled; and every mountain and hill shall be brought low; and the crooked shall be made straight; and the rough ways plain;

A.D. 253 – 1871 9 witnesses 10 excerpts

Patristic

before A.D. 750
253
A.D.
Origen Patristic
c. A.D. 184–253
“For when Jesus had come and sent His Spirit, every valley was filled with good works, and the fruits of the Holy Spirit, which if thou hast, thou wilt not only cease to become a valley, but will begin also to be a mountain of God.”
395
A.D.
Gregory of Nyssa Patristic
c. A.D. 335–395
“(ubi sup.) Or, He orders the valleys to be filled, the mountains and hills to be cast down, to shew that the rule of virtue neither fails from want of good, nor transgresses from excess.”
407
A.D.
John Chrysostom Patristic
A.D. 347–407
“(ubi sup.) He denounces the haughty and arrogant by the name of mountains, whom Christ has brought low. But by the hills He implies the wreckless, not only because of the pride of their hearts, but because of the barrenness of despair. For the hill produces no fruit.”
444
A.D.
A.D. 376–444
“(in Esai. 40. lib. 3.) But suppose some one should answer, saying, How shall we prepare the way of the Lord, or how shall we make His paths straight? since so many are the hindrances to those who wish to lead an honest life. To this the word of prophecy replies, There are some ways and paths by no means easy to travel, being in some places hilly and rugged, in others steep and precipitous; to remove which it says, Every valley shall be filled, every mountain and hill shall he brought low. Some roads are most unequally constructed, and while in one part rising, in another sloping downwards, are very difficult to pass. And here he adds, And the crooked ways shall be made straight, and the rough ways shall be made smooth. But this was in a spiritual manner brought to pass by the power of our Saviour. For formerly to pursue an Evangelical course of life was a difficult task, for men’s minds were so immersed in worldly pleasures. But now that God being made Man, has condemned sin in the flesh, all things are made plain, and the way of going has become easy, and neither hill nor valley is an obstacle to those who wish to advance.”
Source
604
A.D.
Gregory the Great Patristic
c. A.D. 540–604
“(20. in Ev.) Or, the valley when filled increases, but the mountains and hills when brought low decrease, because the Gentiles by faith in Christ receive fulness of grace, but the Jews by their sin of treachery have lost that wherein they boasted. For the humble receive a gift because the hearts of the proud they keep afar off.”
Source
“(ubi sup.) But the crooked places are become straight, when the hearts of the wicked, perverted by a course of injustice, are directed to the rule of justice. But the rough ways are changed to smooth, when fierce and savage dispositions by the influence of Divine grace return to gentleness and meckness.”

Post-Reformation

c. 1650 – 1900
1771
A.D.
John Gill Reformed
1697–1771
“Every valley shall be filled,.... Luke cites more out of the same prophecy, as relating to the times of John the Baptist, and the Messiah, than the other Evangelists Matthew and Mark do: in the prophet it is, "every valley shall be exalted"; which is done, by filling it up; the metaphor is persisted in, of preparing and clearing the way, for the coming of the Messiah, done by the ministry of John; under which, such souls as were lowly and humble, and depressed with the sense of sin, should be raised and directed to believe in Christ, and be filled with divine consolation from him. These words are owned by the Jews (o) to belong to the world to come; that is, the times of the Messiah; though they understand them, of making way for the return of the Israelites from captivity, by the Messiah: just as they suppose such things were done by the miraculous cloud, for the children of Israel, as they passed through the wilderness; of which they say (p), "that it went before them, smote the serpents and scorpions, and fiery serpents, and the rock; and if there was any low place, it raised it up; or high place, it made it low, and caused them to be plain; as it is said, Isa 40:3 "And every valley shall be exalted", &c.'' But what they say of this cloud literally, as preparing the way for the Israelites, is in a spiritual sense true, of the ministry of John; whereby many of the children of Israel, had the way prepared for them, for the reception of the Messiah; when as every humble soul had its expectation raised, and its faith encouraged, and its heart filled with spiritual joy; so such as were proud and haughty, were humbled: and every mountain and hill shall be brought low; all such as are elated with their own abilities, and boast of their righteousness, trust in themselves, and look with disdain and contempt on others, their loftiness shall be bowed down, and their haughtiness made low; and the Messiah alone, in his person, grace, and righteousness, be exalted: and the crooked shall be made straight: such as are of a crooked spirit, and walk in crooked ways, with the workers of iniquity, shall have new spirits given them, and be directed to right ways, and be led in the paths of righteousness and truth: and the rough ways shall be made smooth; and men of rough tempers, comparable to lions and bears, shall become quiet and peaceable, smooth and easy; and moreover, whatever difficulties were in the minds of men concerning the Messiah, the end of his coming, and the nature of his kingdom; and whatever impediments were in the way of embracing him when come, should now be removed at least from many persons: R. David Kimchi, a very noted Jewish commentator (q), acknowledges that the whole of this passage is to be understood, , "by way of parable", in a mystical and figurative sense. (o) T. Hieros. Erubin, fol. 25. 2. (p) Bemidbar Rabba, sect. 1. fol. 177. 1, 2. Vid. Targ. in Cant. ii. 6. & Jarchi in Cant. iii. 6. (q) In Isa. xl. 4.”
Source
1832
A.D.
Adam Clarke Methodist
1762–1832
“Every they shall be filled - All hinderances shall be taken out of the way: a quotation from the Greek version of Isa 40:4, containing an allusion to the preparations made in rough countries to facilitate the march of mighty kings and conquerors. See the instance produced on Mat 3:3 (note).”
1849
A.D.
1774–1849
“Every valley, &c. If these words, in one sense, were a prediction of the deliverance of the Israelites from their captivity, (Isaias xl. 3.) and an admonition to level the roads for those that were to return, they also signified the redemption of mankind from the slavery of sin; and that all obstacles, which retarded this benefit, should be removed, and also that the proud should be depressed, and the humble receive graces. (Witham)”
Source
1871
A.D.
1871
“Every valley, &c.--levelling and smoothing, obvious figures, the sense of which is in the first words of the proclamation, "Prepare ye the way of the Lord."”
Modern · 1953 →

The in-app commentary runs from the Fathers to the early-modern record, then stops — that's where the public-domain sources end, not where the reading does. For the modern reading, follow the sources directly.