From The Gospel of Matthew, by Curtis Mitch and Edward Sri, commenting on Matthew 11:2-11
The third question asks if John was perceived as a prophet. This is a description that fits quite well, yet Jesus declares him to be more than a prophet. Why? Because John not only prophesied the coming of the Messiah but was himself a fulfillment of prophecy.
Jesus demonstrates this by quoting the Old Testament. The citation consists of two separate verses cut and pasted together. The first—Behold, I am sending my messenger ahead of you—is taken verbatim from the †Septuagint translation of Exod 23:20.5 The second—he will prepare your way before you—is a rendering of the Hebrew text of Mal 3:1.6 Both passages tell us that John played a crucial role in readying Israel for the Messiah.
The Exodus quote, which concerns Israel’s march to the land of Canaan, suggests that John was leading Israel to a new promised land of messianic redemption. The oracle of Malachi indicates that John was preparing his people to meet the Lord. In fact, the context of Malachi’s declaration suggests that the one who prepares the way will be Elijah come again (see Mal 3:23).
Matthew’s readers will not be surprised to hear this, for they have already seen John dressed like Elijah (compare 3:4 with 2 Kings 1:8), and they first encountered John at the Jordan River, which is the very place where Elijah was last seen centuries earlier (compare 3:6 with 2 Kings 2:6–12). But just in case these hints are too subtle, Jesus comes out and tells us that John is Elijah, the one who is to come. The point is not that John is Elijah himself but that his prophetic mission is like Elijah’s. Luke’s Gospel captures this by saying that John comes “in the spirit and power of Elijah” (Luke 1:17).