J. A. Alexander and the mountains of Isaiah 40:4
Isaiah 40:3-5 appears to show that the glory of the Lord will be revealed by the mountains being brought low, and the rough places being made smooth, and the valleys filled. But this must be explained; the prophet was not speaking of literally making the mountains low.
In his commentary on Isaiah, Joseph Addison Alexander struggled to explain the thought underlying Isaiah’s prophecy. He suggested that Isaiah’s imagery refers to making a way in the wilderness, removing obstacles, which would allow God to revisit his people. Alexander wrote on verses 3-5: 
3. A voice crying—in the wilderness—clear the way of Jehovah —make straight (or level) in the desert a highway for our God. The Septuagint version, retained in the New Testament, is …, (the voice of one crying) which amounts to the same thing. Both in the Hebrew and the Greek, the words in the wilderness may be connected either with what follows or with what precedes; but the usual division is more natural, and the other has been insisted upon chiefly for the purpose of rendering the verse inapplicable to John the Baptist, who came preaching in a wilderness, and to whom the words are applied expressly in Matthew 3:3. Mark 1:3, Luke 3:4, as the herald of the new dispensation. Those who deny the inspiration of the Prophet are compelled to reject this as a mere accommodation, and apply the verse exclusively to the return from Babylon, of which there is no mention in the text or context. It is said indeed that God is here represented as marching at the head of his returning people. But in all the cases which Lowth cites as parallel, there is express allusion to the exodus from Egypt. Here, on the contrary, the only image presented is that of God returning to Jerusalem, revisiting his people, as he did in every signal manifestation of his presence, but above all at the advent of Messiah and the opening of the new dispensation. The verb rendered prepare denotes a particular kind of preparation, viz. the removal of obstructions, as appears from Gen. 24:31, Lev. 14:36, and may therefore be expressed by char in English. The parallel verb means rectify or make straight, either in reference to obliquity of course or to unevenness of surface, most probably the latter, in which case it may be expressed by level. To a general term meaning way or path is added a specific one denoting an artificial causeway raised above the surface of the earth. There is no need of supposing that the Prophet here alludes to any particular usage of the oriental sovereigns, or that the order of the first and second verses is continued (let there be a voice crying). The Prophet is describing what he actually hears—a voice crying!—or, hark, one cries!
4. Every valley shall be raised and every mountain and hill brought low, and the uneven shall become level and the ridges a plain. This may be considered as an explanation of the manner in which the way of the Lord was to be prepared. The common version (exalted) seems to imply that the valleys and mountains were to exchange places; but this would not facilitate the passage, which requires that both should be reduced to a common level. The whole impression here intended to be made is that of a way opened through a wilderness by levelling the ground and the removal of obstructions, as a natural image for the removal of the hinderances to God’s revisiting his people.
5. And the glory of Jehovah shall be revealed, and all flesh shall see (it), for the mouth of Jehovah speaks. (or hath spoken). The idea seems to be that as soon as the way is opened, the Lord will show himself. To see God’s glory, is a common expression for recognizing his presence and agency in any event. (See Exod. 16 : 7. Is. 35 : 2. 66: 18.) The specific reference of this verse to the restoration of the Jews from exile is not only gratuitous but inconsistent with the strength and comprehensiveness of its expressions. The simple meaning is, that when the way should be prepared, the glory of God would be universally displayed; a promise too extensive to be fully verified in that event or period of history.
Alexander did not grasp the full meaning of Isaiah’s prophecy, as he seems to have misunderstood the symbolic meaning of mountains. Unless the metaphor of mountains, and rough places, and of the land itself, is fully understood, the prophecy of Isaiah seems vague. Why would God need a road built in the desert? But, in another place in his commentary, where he commented on a prophecy closely related to 40:4, Alexander provided a glimpse of what the land means. He commented on the following verses:
When thou criest, let thy companies deliver thee; but the wind shall carry them all away; vanity shall take them: but he that putteth his trust in me shall possess the land, and shall inherit my holy mountain; And shall say, Cast ye up, cast ye up, prepare the way, take up the stumblingblock out of the way of my people.
On these verses Alexander wrote: 
13. In thy crying (i e. when thou criest for help), let thy gatherings save thee! And (yet) all of them the wind shall take up and a breath shall take away, and the (one) trusting in me shall inherit the land and possess my holy mountain. This is merely a strong contrast between the impotence of idols and the power of Jehovah to protect their followers respectively. Some understand the word translated gatherings generically, as denoting all that they could scrape together for their own security, including idols, armies, and all other objects of reliance. Those who restrict the passage to the Babylonish exile must of course explain the promise as relating merely to the restoration; but the context and the usage of the Scriptures is in favour of a wider explanation, in which the possession of the land is an appointed symbol of the highest blessings which are in reserve for true believers here and hereafter.
14. And he shall say, Cast up, cast up, clear the way, take up the stumbling-block from the way of my people! He who had long been silent speaks at last, and that to announce the restoration of his people. The image here presented, and the form of the expression, are the same as in ch. 35:8, 40:3, 49:11, 62:10.
Alexander’s statement that the land promise relates to “a wider explanation, in which the possession of the land is an appointed symbol of the highest blessings which are in reserve for true believers here and hereafter” is correct, and if this idea is followed a little further, it leads to the better understanding of mountains and rough places in 40:4. Because the mountains are high, and are prominent landmarks, and also durable, the mountains of the promised land represent high and lofty spiritual blessings or promises to the saints. The literal mountains have been made low, because they were mere types and shadows of those true spiritual blessings, which include understanding the prophecies. According to Isaiah 40:5, the glory of the Lord is revealed, and all flesh will see it, as these mysteries are understood by the saints.
Isaiah 57:14 refers to removing stumbling-blocks, and clearing the way, which is similar to the idea in 40:4. The meaning is spiritual; the spiritual stumbling-blocks need to be removed, including the flawed literal interpretations of prophecy! Then God’s glory is seen by all flesh.
1. J. A. Alexander, Isaiah translated and explained. Volume 2. John Wiley, NY. 1851. p p. 72-74.