Matthew 3:3

For this is he that was spoken of through Isaiah the prophet, saying, The voice of one crying in the wilderness, Make ye ready the way of the Lord, Make his paths straight.

The passage here quoted is Isaiah 40:3, but Isaiah is not the only prophet who prophesied of the mission of the great herald of the gospel age. Another equally significant prophecy is Malachi 4:5,6 in which the office and work of the herald are explicitly foretold. It is from Christ himself that one learns this passage's application to John. Jesus had identified himself as the Messiah to his disciples, but the disciples had been troubled by the objection of the scribes that "Elijah must first come" (Matthew 17:9-13). Christ then identified John as the "Elijah" foretold by Malachi. The Pharisees should have known that truth already, because it was to one of the priestly group, Zacharias, that the angel announced the birth of John, using almost the identical words of Malachi's great prophecy. Compare Malachi 4:5,6 and Luke 1:15-17. Only willful blindness on the part of the Jewish leaders can account for their failure to recognize John as the "Elijah" who was to precede the Messiah.