I have often written here about the fascinating yet enigmatic person of John the Baptist, who is presented as an extremely important figure (second only to Jesus) in the Fourth (and other) Gospels.
But the nature of John's importance, the deep reasons why he was crucially important; are, by my judgement, poorly explained in the sources I have encountered.
I have previously suggested several explanations of John's importance - but now I think I have finally reached to the bottom of the matter!...
The key is that Jesus was only potentially the Messiah until John recognised then baptised him.
So, Jesus was already a sinless Man, perfectly aligned with God's plan and purposes, 'destined' from before his incarnation to be the Messiah; but as such, Jesus could not recognise himself as the Messiah.
Jesus needed to be told that he was the Messiah - and he needed to be told by a person of authority, discernment and total honesty: that is by a true prophet.
John was the greatest religious figure of his day, universally respected and revered, probably the only acknowledged Hebrew prophet for hundreds of years. John was uniquely qualified to recognise Jesus as the Messiah, and to tell him and be believed.
Only then could Jesus actively choose to embrace his destiny; and he did so by requesting baptism from John. We could say that, at the moment of baptism, Jesus (as an adult, of supreme intelligence and scholarly knowledge) made a fully 'informed' decision now to become who he already-was potentially.
And at this moment, John saw the spirit descend upon Jesus and stay upon him: at the baptism Jesus became divine.
And John's work was completed.
John 1:  The next day John seeth Jesus coming unto him, and saith, Behold the Lamb of God, which taketh away the sin of the world.
 This is he of whom I said, After me cometh a man which is preferred before me: for he was before me.
 And I knew him not: but that he should be made manifest to Israel, therefore am I come baptizing with water.
 And John bare record, saying, I saw the Spirit descending from heaven like a dove, and it abode upon him.
 And I knew him not: but he that sent me to baptize with water, the same said unto me, Upon whom thou shalt see the Spirit descending, and remaining on him, the same is he which baptizeth with the Holy Ghost.
 And I saw, and bare record that this is the Son of God.