Here's another quote that has been helpful in understanding the ministry of Jesus in Mark's Gospel.
Mark 1:9-11 (ESV)
In those days Jesus came from Nazareth of Galilee and was baptized by John in the Jordan. And when he came up out of the water, immediately he saw the heavens being torn open and the Spirit descending on him like a dove. And a voice came from heaven, “You are my beloved Son; with you I am well pleased.”
James R. Edwards, in his fine commentary on the Gospel of Mark, remarks that,
The baptism is the keystone in the life and ministry of Jesus. The empowerment by the Spirit to be God’s Servant, and the declaration from heaven, “‘You are my Son,’” enable Jesus not only to speak and act for God but as God. This is demonstrated by his forgiveness of sins (2:5), acceptance of sinners (2:15), calling of tax collectors into discipleship (2:13), true intent of the Sabbath (2:28) and challenge to the Jewish religious establishment as represent in the oral tradition (7:1ff), the temple (11:12ff), and the Sanhedrin (14:61ff). It is not coincidental that when Jesus is later confronted by the Sanhedrin asking, “By what authority do you do these things?” he drives his questioners back to his baptism (11:27-33). What Jesus does as God’s servant ultimately has meaning only because of who he is as God’s Son. (The Gospel According to Mark. Pillar New Testament Commentary, p. 38)
Interesting, Jesus' baptism is not about what he does, but rather about what the Father and Spirit say and do. The Father declares his love and discloses their relationship. The Spirit fills and equips Jesus for service.
That this event is the keystone to the life and ministry of Jesus is also indicated by the fact that the apostles seek for a replacement for Judas among those who were witnesses of Jesus' earthly ministry "beginning from the baptism of John" (Acts 1:21-22; cf. Edwards, p. 34).