Before Jesus came to be baptized, John proclaimed Him as a mysterious Person: “He who is coming after me is mightier than I … He will baptize you with the Holy Spirit and with fire.”
At some point, I imagine John asking, “Lord, You sent me on this mission to prepare Your people for the Messiah. When He arrives, how can I be sure of Him so I can say to the people ‘This is the One I’ve been telling you about’?” And God said:
… ‘He on whom you see the Spirit descend and remain, this is he who baptizes with the Holy Spirit.’
– John 1:33b (ESV)
Perhaps John thought, “I’m going to see the invisible Spirit of God descend on the Messiah? This should be interesting!”
When Jesus came, John recognized Him as the One he was looking for (Matthew 3:14), and yet he did not know Him (John 1:33, greek oida). John believed, but it was not the inward, conscious certitude of the heart. (I think the plea in Mark 9:24 echoes this: “I believe, help my unbelief!”). John needed supernatural revelation to be certain that Jesus is the Anointed One.
…And when Jesus was baptized, immediately he went up from the water, and behold, the heavens were opened to him, and he saw the Spirit of God descending like a dove and coming to rest on him; and behold, a voice from heaven said, “This is my beloved Son, with whom I am well pleased.”
– Matthew 3:16-17 (ESV)
It happened just as God had said, with a couple extra convincers thrown in for good measure: the heavens opened, the Holy Spirit came down in visible form and remained on Jesus, and God gave His utterly incontrovertible testimony: “This is my beloved Son. I am fully delighted with Him.” It would take an act of will (or “won’t”) to doubt now.
Those who had previously heard John’s testimony had now witnessed a greater. They even witnessed fulfillment of prophecy, for Isaiah wrote:
There shall come forth a shoot from the stump of Jesse, and a branch from his roots shall bear fruit. And the Spirit of the LORD shall rest upon him, the Spirit of wisdom and understanding, the Spirit of counsel and might, the Spirit of knowledge and the fear of the LORD. And his delight shall be in the fear of the LORD. He shall not judge by what his eyes see, or decide disputes by what his ears hear, but with righteousness he shall judge the poor, and decide with equity for the meek of the earth; …
– Isaiah 11:1-5 (ESV)
Behold my servant, whom I uphold, my chosen, in whom my soul delights; I have put my Spirit upon him; he will bring forth justice to the nations.
– Isaiah 42:1 (ESV)
When the Spirit came down on Jesus, it was not just some theatrical effect designed to create a sense of awe in the onlookers. The Spirit’s visible presence on Jesus confirmed that He would be the One to baptize with the Holy Spirit.
Now John knew the One he was proclaiming. From this point on, whenever he saw Jesus, he said, “This is the Son of God” (vs. 34), “Look! The Lamb of God who takes away the sin of the world!” (vs 1:29, 36).
The Holy Spirit also served another purpose here. While John prepared Jesus by baptizing Him in water, it was the Holy Spirit who really prepared Jesus for His ministry. The Spirit never left Him. For the next 3 years, Jesus would minister not in His own power but in that of the Spirit (Matthew 12:28, Acts 10:38).
These two things, the testimony and preparation of the Holy Spirit, apply to us as followers of Jesus.
First, just as at His baptism, the full knowledge of who Jesus is comes about through the testifying work of the Holy Spirit. We can acquire knowledge of Jesus through the scriptures and come to believe in a way, but full knowledge of Him always takes special revelation from God (John 16:12-14, 1 Corinthians 2:1-16, 12:3). I need the Spirit to testify to me about Jesus, and I need the Spirit to help me tell others about Him.
Secondly, we need the Holy Spirit to prepare us for ministry. While Jesus certainly had the power and ability to minister in His own power, we don’t have the choice. And even if we did, since our Lord chose to walk in the Spirit, we need to do the same. There is no other way to follow Him.