from Pope Benedict's Jesus of Nazareth, volume one: From the Baptism in the Jordan to the Transfiguration:

Luke tells us that Jesus was praying while he received baptism (cf. Lk 3:21). Looking at the events in light of the Cross and Resurrection, the Christian people realized what happened: Jesus loaded the burden of all mankind's guilt upon his shoulders; he bore it down into the depths of the Jordan. He inaugurated his public activity by stepping into the place of sinners. His inaugural gesture is an anticipation of the Cross. He is, at it were, the true Jonah who said to the crew of the ship, "Take me and throw me into the sea" (Jon 1:12). The whole significance of Jesus' Baptism, the fact that he bears "all righteousness," first comes to light on the Cross. The Baptism is an acceptance of death for the sins of humanity, and the voice that calls out, "This is my beloved Son," over the baptismal waters is an anticipatory reference to the Resurrection. This also explains why, in his own discourse, Jesus uses the word baptism to refer to his death (cf. Mk 10:38; Lk 12:50). (p. 18)Luke does not place his genealogy of Jesus at the beginning of the Gospel, but connects it with the story of Jesus' Baptism, to which it forms a conclusion....In contrast to Matthew, Luke uses his genealogy to journey from Jesus back into past history. Abraham and David make their appearance, but without any particular emphasis. The family tree goes back to Adam, and so to creation, for once Luke comes to the name Adam, he adds: "of God." This is a way of underscoring the universal scope of Jesus' mission. He is the son of Adam -- the son of man. Because he is man, all of us belong to him and he to us; in him humanity starts anew and reaches its destiny. (p.10)

Giovanni Bellini's great altarpiece of the Baptism of Jesus: