So what makes the baptism of Jesus so meaningful and significant? The answer can be found in the Gospels, particularly in the Gospel of John:
 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.
In his homily today, my parish priest emphasized the pivotal nature of the John's baptism of Jesus by stating that it signaled a divine transformation, the significance of which should not be underestimated or undervalued. The transformation Jesus undergoes during baptism is an event Bruce Charlton has also written extensively about on his Notions Blog over the years. He encapsulated the significance of the baptism very effectively in a recent post from which the excerpts below have been drawn:
John (the Baptist) seems to have baptised hundreds of people before Jesus; and he had the ability to call down the spirit of God so that it would 'touch' each person for a moment. But when he baptised Jesus, the spirit remained - and that was when Jesus underwent the 'extraordinary transformation' and became fully divine as a mortal man.
This shows the uniqueness of Jesus; that there was something about him which none of the other hundreds who were baptised had - and which John himself lacked. It shows that if we have sin (that is, if we are not fully aligned with God's motivations and purposes) the spirit of God can do no more than touch us; and this state of sin is universal - except for Jesus.
Seen in this light, John's baptism of Jesus is far more than a prelude, or an introduction, or a minor subplot. It is a key event - not only within the biblical narrative, but also in the history of the world.