Matthew 3:3

For this is he that was spoken of by the prophet Esaias, saying-- (Mat 11:3). The voice of one crying in the wilderness--(See on Luk 3:2); the scene of his ministry corresponding to its rough nature. Prepare ye the way of the Lord, make his paths straight--This prediction is quoted in all the four Gospels, showing that it was regarded as a great outstanding one, and the predicted forerunner as the connecting link between the old and the new economies. Like the great ones of the earth, the Prince of peace was to have His immediate approach proclaimed and His way prepared; and the call here--taking it generally--is a call to put out of the way whatever would obstruct His progress and hinder His complete triumph, whether those hindrances were public or personal, outward or inward. In Luke (Luk 3:5-6) the quotation is thus continued: "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 shall be made smooth; and all flesh shall see the salvation of God." Levelling and smoothing are here the obvious figures whose sense is conveyed in the first words of the proclamation--"Prepare ye the way of the Lord." The idea is that every obstruction shall be so removed as to reveal to the whole world the salvation of God in Him whose name is the "Saviour." (Compare Psa 98:3; Isa 11:10; Isa 49:6; Isa 52:10; Luk 2:31-32; Act 13:47).

— Jamieson-Fausset-Brown Commentary (public domain)