7 “And they asked him, What then? Are you Elias? And he said, No. And they said unto him, Are you a prophet? And he answered, No! They said therefore unto him, Who are you? That we may give an answer to them that sent us. What do you say of yourself? He says, I am the voice of one crying in the wilderness.” That said Isaiah. This prophecy was fulfilled in John, “I am the voice of one crying in the wilderness.” Crying what? “Prepare ye the way of the Lord, make straight the paths of our God.” Would it not have seemed to you that a herald would have cried, “Go away, make room.” Instead of the herald's cry “Go away,” John says “Come.” The herald makes men stand back from the judge; to the Judge John calls. Yes, indeed, John calls men to the lowly One, that they may not experience what He will be as the exalted Judge. “I am the voice of one crying in the wilderness, Prepare ye the way of the Lord, as said the prophet Isaiah.” He did not say, I am John, I am Elias, I am a prophet. But what did he say? This I am called, “The voice of one crying in the wilderness, Prepare the way for the Lord: I am the prophecy itself.”
Source: Tractates on the Gospel of John (New Advent)