‘As it is written in the book of the words of Isaiah the prophet.
a “The voice of one crying in the wilderness,
b Make you ready the way of the Lord,
c Make his paths straight.
d Every valley shall be filled,
d And every mountain and hill shall be brought low,
c And the crooked shall become straight,
b And the rough ways smooth.
a And all flesh shall see the salvation of God.
Note the chiastic structure here. In ‘b' and parallel we have the play on ‘way', in ‘c' and parallel on ‘smooth', while ‘d' and parallel are identical but contrasting thoughts. This is as well as the parallels of the poetic rhythm.
The above is fairly similar to (but not the same as) Isaiah 40:3-5 LXX, ‘the voice of one crying in the wilderness, Prepare the way of the Lord, make straight the paths of our God, every valley will be filled, and every mountain and hill will be brought low, and all the crooked ways will become straight, and the rough places plains, and the glory of the Lord will appear, and all flesh will see the salvation of God, for the Lord has spoken'. But as findings at Qumran remind us, the differences probably indicate that it was not a direct citation from there, and could easily rather be a translation from an original Hebrew text. (This is true wherever there is a similarity to LXX but with differences). Luke, however, like us, would use any suitable versions that were available.
It will be noted that ‘and all flesh will see the salvation of God' is not present in the Hebrew text, which reads ‘and the glory of the Lord will be revealed and all flesh shall see it (the glory of the Lord) together'. But the whole of the remainder of Isaiah 40 demonstrates that the revealing of the glory of the Lord there is in fact for the purpose of salvation (Luke 3:10-11; Luke 3:28-31), and thus that those who see His glory will see His salvation, for His glory will be revealed in salvation. Thus the significance is the same.
Luke is here following the example of Mark 1:3, but expanding on it, and cites with regard to John the words of Isaiah in Isaiah 40:3-5. These words referred to one who was to prepare the way for God to act. (The community at Qumran applied this verse to their task of studying the law in the wilderness). It was normal in Isaiah's day for the approach of great kings to be prepared for by smoothing the way before them. There were to be no ups and downs for a great king. The valleys would be filled in, the mountains levelled, the road would be made straight and all obstacles and unevenness removed. And both Isaiah and John saw this in spiritual terms, the removal of the obstacles of men's sins and the preparation of men's hearts ready for God to act.
We can compare how later men are to be ‘brought low' (Luke 14:11; Luke 18:14), for good or bad, and the crooked will be made straight (Luke 13:10-17, the Greek terms are different but have the same significance) which was itself a picture of what Christ had come to do. For the mountains being levelled we can compare Zechariah 4:7 where such an event is connected with the Spirit and refers to the removal of all obstacles on behalf of Zerubbabel. And we can compare how today we speak of the removal of a person's rough edges in order (in our view) to make him a more fulfilled person. The word used for ‘crooked' occurs in a moral sense in Acts 2:40; Philippians 2:15; 1 Peter 2:18, where the idea is of perverseness, or of being overbearing. The word for ‘smooth' is used of walking morally in Proverbs 2:20 LXX where the way for the righteous man is to be smoothed. Thus the words are indicating the total transformation of men and women as explained in Luke 1:17.
‘The voice of one crying in the wilderness.' It has already been made clear that John's preparation has taken place in the wilderness (Luke 1:80; Luke 3:2 and compare Luke 7:24). The wilderness was in Jewish tradition the place where men could go and meet God. We can compare Moses (Exodus 3:1-6) and Elijah (1 Kings 19:4-18) who both met God in the wilderness and received His word there. And John too had met God there and received His word (Luke 3:2).
‘All flesh will see the salvation of God.' We may interpret this as being explained by the words of Simeon in Luke 2:30, ‘my eyes have seen the Salvation of God', thus meaning that all kinds of men (all flesh) will see in Him their Saviour. All kinds and classes will become aware of God's message of salvation. Of course in the end seeing salvation and seeing the Saviour both mean the same thing for God's salvation is only found through Christ.
But the thought might be that all flesh everywhere will see His salvation, some to their benefit and others to their dismay (Revelation 1:7). For when His glory is revealed and His salvation comes, the elect will be gathered in (Matthew 24:31) and the remainder will be subjected to judgment.