For Mark, the gospel of Jesus Christ begins with Jesus' baptism. Before baptism there is no gospel, and without baptism there would be no gospel. Jesus enters the waters as Jesus of Nazareth, and emerges from them as the Son of God. In baptism God claims Jesus as God's own beloved Son, and calls him from those waters to live out his identity. Today is the day for remembering that the same is true for us.
Baptism begins the story of the church.
Baptism is the sign and seal our identity, of our belonging to God. In these waters God calls us by name, and brands us as God's own. These waters dissolve all pretension and pomposity. They strip away all claims to wisdom or righteousness or knowledge. Our forebears in the faith came naked to these waters, and, spiritually speaking, we do the same.