John appears as the boundary between the two testaments, the old and the new. That he is a sort of boundary the Lord himself bears witness, when he speaks of “the law and the prophets up until John the Baptist.” Thus he represents times past and is the herald of the new era to come. As a representative of the past, he is born of aged parents; as a herald of the new era, he is declared to be a prophet while still in his mother’s womb. For when yet unborn, he leapt in his mother’s womb at the arrival of blessed Mary. In that womb he had already been designated a prophet, even before he was born; it was revealed that he was to be Christ’s precursor, before they ever saw one another. These are divine happenings, going beyond the limits of our human frailty.
When John was preaching the Lord’s coming he was asked, “Who are you?” And he replied: “I am the voice of one crying in the wilderness.” The voice is John, but the Lord “in the beginning was the Word.” John was a voice that lasted only for a time; Christ, the Word in the beginning, is eternal.
Unfortunately, I don't yet have the citation info for this quote. For all I know it could be from the same sermon I quoted below (Sermo 293).
UPDATE: Well, I was right. Thanks to Zadok the Roman, I now know this quote comes from Augustine's Sermo 293, 1-3. Ah, the Internet.