The Messiah would be preceded by a forerunner
As Malachi 3:1 does (see commentary on that verse), Isaiah 40:3-5 speaks of someone coming to prepare God’s way. Where Malachi talks about a messenger coming to bring God back to His Temple, Isaiah speaks of a voice crying out to prepare God’s way in the desert. Here is the passage in Isaiah:
A voice cries: “In the wilderness prepare the way of the Lord; make straight in the desert a highway for our God. Every valley shall be lifted up, and every mountain and hill be made low; the uneven ground shall become level, and the rough places a plain. And the glory of the Lord shall be revealed, and all flesh shall see it together, for the mouth of the Lord has spoken.”
This passage envisions God returning to the land of Israel from afar, with the travel preparations suitable for a king. As Malachi implies, here Isaiah is explicit: God’s glory will be revealed.
Matthew 3:1-3 links this passage with John the Baptist, and specifically with the fact that John ministered in the desert:
In those days John the Baptist came preaching in the wilderness of Judea, “Repent, for the kingdom of heaven is at hand.” For this is he who was spoken of by the prophet Isaiah when he said, “The voice of one crying in the wilderness: ‘Prepare the way of the Lord; make his paths straight.’”
There had already been a Jewish community living in the desert (known as the Essenes or the Qumran community); now John appears there, and he attracted enough attention that people came out to see him.
In the fifteenth year of the reign of Tiberius Caesar, Pontius Pilate being governor of Judea, and Herod being tetrarch of Galilee, and his brother Philip tetrarch of the region of Ituraea and Trachonitis, and Lysanias tetrarch of Abilene, during the high priesthood of Annas and Caiaphas, the word of God came to John the son of Zechariah in the wilderness. And he went into all the region around the Jordan, proclaiming a baptism of repentance for the forgiveness of sins. As it is written in the book of the words of Isaiah the prophet, “The voice of one crying in the wilderness: ‘Prepare the way of the Lord, make his paths straight. Every valley shall be filled, and every mountain and hill shall be made low, and the crooked shall become straight, and the rough places shall become level ways, and all flesh shall see the salvation of God.’”
Like Matthew, Luke also mentions that John is in the desert, though he specifies “all the region around the Jordan,” and he also gives the content of his message. For how is the way of the Lord prepared? By repenting of sins. And John was “proclaiming” (like the voice crying in Isaiah 40:3) a baptism of repentance. Though Judaism of the time had various ritual washings, non-Jews who converted to Judaism underwent an immersion. John may be saying that even Jewish people need this kind of baptism to prepare for God; Gentiles and Jews stand equally in need of the same forgiveness.1
In John’s Gospel, John the Baptist himself says that he is the fulfillment of Isaiah 40:3-5:
So they [a delegation of priests and Levites] said to him, “Who are you? We need to give an answer to those who sent us. What do you say about yourself?” He said, “I am the voice of one crying out in the wilderness, ‘Make straight the way of the Lord,’ as the prophet Isaiah said.”
John was being asked about his identity by the priestly delegation sent to meet him in the desert. He had already told them that he was neither the Messiah, nor Elijah the prophet (see commentary on Malachi 4:5-6), nor the prophet of Deuteronomy 18. But they had to bring back a report on John, so they continue to press him. That is when he quotes Isaiah 40:3. They still press on: why, they ask, are you baptizing anyone if you aren’t the Messiah or Elijah or the Prophet? It could be that they expected one of those end-times figures to baptize people as an act of repentance. John’s response is to point beyond himself to Jesus, though he does not mention him by name here. He is merely the one who prepares, John tells them. But wait till you see who I am preparing things for!