Catena Aurea on John 1:19–23: Gregory the Great

But if we examine the truth accurately, that which sounds inconsistent, will be found not really so. The Angel told Zacharias concerning John, He shall go before Him in the spirit and power of Elias. (Luke 1:17) As Elias then will preach the second advent of our Lord, so John preached His first; as the former will come as the precursor of the Judge, so the latter was made the precursor of the Redeemer. John was Elias in spirit, not in person: and what our Lord affirms of the spirit, John denies of the Person: there being a kind of propriety in this; viz. that our Lord to His disciples should speak spiritually of John, and that John, in answering the carnal multitude, should speak of his body, not of his spirit.