Isaiah 40:5

and the glory of the Lord shall be revealed, namely, chiefly in the coming of His only-begotten Son, full of grace and truth, John 1:14, and all flesh shall see it together, all men having an opportunity to become acquainted with the Gospel of salvation, Matthew 24:14; for the mouth of the Lord hath spoken it. The paragraph deals with the call to repentance with which the Messianic era opened, particularly in the preaching of John the Baptist, Matthew 3:1; Mark 1:2; Luke 3:2; John 1:23, but finds its application also in all true preaching of the New Testament, which is essentially a proclamation of sin and grace. The prophet now introduces a dialog between a heavenly voice and that of the ideal prophet concerning the proclamation which is to be made.