All four gospels give an account of the baptism of Jesus by John the Baptist. Mark begins his gospel with the baptism of Jesus and John bears witness to his identity through the Baptist, who recounts seeing the Spirit descending on him. All four gospels point to this event as the beginning of the public life of Jesus.
The Jordan had been the river that the Israelites crossed miraculously after their forty years in the wilderness, before they began their new life in the Promised Land (Joshua 3). Naaman, the Syrian, was cured in its waters (2 Kings 14). Jesus would restore new life to mankind and his ministry would bring healing to his people.
He had experienced a very ordinary upbringing in Nazareth with Mary and Joseph. He would have been well known as the village carpenter; now he was entering a life where he would have no home of his own, would constantly be in the public eye and would soon upset the establishment figures of the time, as John the Baptist had already done. He must have felt deeply the affirmation of the Father and the Spirit, as he emerged from the waters of the Jordan.
The three persons of the Trinity descend on us at our Baptism and we are clasped in the radiance of their circle of love.
The evangelist John is a skilled dramatist. Jesus is of course the central figure, but the other characters such as the Baptist, Mary his mother (not mentioned by name), Nicodemus and the Woman of Samaria are brought into an encounter with Jesus so that his identity is revealed in his interactions with them.