John 1:32

And John bare witness, saying, I have beheld the Spirit descending as a dove out of heaven; and it abode upon him. And I knew him not: but he that sent me to baptize in water, he said unto me, Upon whomsoever thou shalt see the Spirit descending, and abiding upon him, the same is he that baptizeth in the Holy Spirit.

In order for John to be able to see the Holy Spirit, it was necessary for the Spirit to assume a physical form; and, appropriately, it was that of a dove, long the symbol of peace and goodness. For a fuller comment on this symbolism of the dove, see my Commentary on Matthew, Matthew 3:16. Of course, this was not a mere case of a bird lighting on Jesus for a moment, a phenomenon which, while rare, is occasionally experienced by men. Much more was involved. The heavens were opened, and the dove visibly descended from on high, an action totally dissociated from the invariable flight pattern af a dove, which is always horizontal. Also, there was a voice out of heaven (Matthew 3:17), the same being the testimony of God himself that "This is my beloved Son in whom I am well pleased." In addition to all this, the Spirit-dove remained visibly upon the Lord. Thus, Jesus was absolutely identified as the one who would baptize in the Holy Spirit, with the necessary deduction that the same was the Messiah. For discussion on the subject of baptism, see my Commentary on Matthew. Matthew 3:11, and my Commentary on Hebrews, Hebrews 6:1,2.

Without the witness of this Gospel, people might never have known how John the Baptist arrived at the conviction that Jesus was indeed the Christ. The apostle John, one of the great herald's disciples at first, was enlightened on this by John himself, and was, moreover, an eye-witness of the fulfillment of the key prophecy of identification.