Every good story, including each of the Gospels, begins with some sort of call to action. Things don’t start with the actual ministry of Jesus, though that is their primary focus, but start with some other action. For Matthew it’s the birth of Jesus and so the action is on his parents. For Luke, it’s the birth of John the Baptizer. For John it’s the creation of the world and conflict between light and dark. Mark begins with the action. The actions of John the Baptizer and the establishment of the Kingdom of God.
John the Baptizer knows his role as one making preparation, not establishing the kingdom. His reference is to the book of Isaiah, where the prophet gives a message of hope for a future he will never see. They are to prepare a way in the dessert for a return to their homeland for exile. The Gospel of Mark, then, accepts that, though the people of Israel are living in their geographic home, they are still living in exile. Rome is in charge and they are non-citizen subjects. Herod is a farce of a king. In general the people have little hope. And yet, John cries out to prepare for a change. God is not alien to you, but is coming to redeem you again, to bring you back to his self.
Then, he arrives. He is baptized in the river and we see Father, Son, and Spirit all at once as Jesus comes out of the water. Change is starting. Things will never be the same. “The time has come” declares John. “The Kingdom of God is near.” That is certainly good news.