Behold, I will send you Elijah

The Gospel according to Matthew 3:1-6

At that time, 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." Now John wore a garment of camel's hair, and a leather girdle around his waist; and his food was locusts and wild honey. Then went out to him Jerusalem and all Judea and all the region about the Jordan, and they were baptized by him in the river.

Icon of Ss. Elijah and John the Baptist
Sourced from modeoflife.org

The significance of the person of John at this point in the Gospel as we approach the feast of the Theophany is integral to the continuity of the story of salvation of God's people.
Christ does not come unannounced to his people, he is foretold by the ancient prophets and prefigured in the final of those, John, the Baptist, Our Lord's cousin.

Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the Lord. (Malachi 4:5)

The description Matthew gives us of John's attire is not without significance. The camel hair and leather belt refers John to Elijah, it is the dress code of the Prophet of the Lord.

"What kind of man was he who came to meet you and told you these things? They answered him, "He wore a garment of haircloth, with a girdle of leather about his loins." And he [King Ahazi'ah] said, "It is Elijah the Tishbite." (2 Kings 1:7-8)

 And so with John comes the completion of the Old Prophets, and the beginning of the era of the Incarnate God. In Christ history is fulfilled and perfected, in the incarnation the requirement for prophets to reveal God's will among men was no longer needed for Christ is with us intrinsically through the flesh.

With the coming of Christ, the ministry of John came to completion, the new age begins, and the old rites perfected.

I baptize you with water [...] he will baptize you with the Holy Spirit and with fire (Matthew 3:11)