Great expectations have always been part of the human story.
Charles Dickens wrote an entire book with that as its title. Anyone can relate to looking forward to or anticipating something. In Luke's Gospel, the people were expecting a Messiah and wondering if John the Baptist could be he. Luke and the reader both know he is not. Therefore, the mention of messianic expectation is ironic because the Messiah does appear in the next few verses. However, his actual appearance is wholly unexpected as is his redefinition of "Messiah" later on.
Luke has built up the expectations in his Gospel thus far by relating the pre-history of Jesus, his birth, early trip to Jerusalem, and the appearance of the Baptist. In this passage, however, the expectations of the people are shattered without their knowing it. Whereas they had probably been expecting an apocalyptic messiah figure that would restore their political fortunes, they got an apocalyptic figure who redefined apocalypse, the messiah, and their expectations.
First, the Baptist dampens the anticipations of his listeners when he mentions the "wrath to come" in the preceding verses. He is by no means gentle with his listeners when he predicts fire for those trees that bear no fruit. If they thought that John was looking forward to a serious drubbing of the Romans or other traditional enemies of the Jews, they would be sorely disappointed. He is more concerned with personal spiritual transformation within Israel itself.
At this point, Luke explicitly mentions expectations. He is the only evangelist to do so. In 1:21, the people expect Zachariah to emerge from the temple and speak. Later in chapter 7, the Baptist asks Jesus if he is indeed the expected one. Each time the result or the answer is a surprise. Zachariah cannot speak and the people know that he has seen a vision. John receives an enigmatic reply from Jesus, as though the question were absurd. The reply in this story is both more subtle and more powerful. It is subtle in that it is not what the crowd expected nor was the reply meant for them. It is powerful in that the reply comes from an impeccable source. All human expectations will be redefined in this Gospel.
John first replies to the expectations of the people by telling them that someone greater than he is coming. This message is shared by all three synoptic Gospels, but the reply concerning the threshing floor occurs only in Matthew and Luke. The Baptist mentions the baptism of the Holy Spirit and fire. Perhaps this is a reference to Pentecost in Acts 2. Perhaps it is a reference to Jesus' powerful life and teachings. The preacher may want to make the point that every baptism in the Lutheran church is a baptism of the Holy Spirit. That is why it is taken so seriously. It is a miracle each time it occurs.
The seriousness of baptism is made clear by the metaphor of the threshing floor. It is a discriminating rite. It is not an act that one may undergo lightly, but is linked to salvation in opposition to judgment. The Holy Spirit is not inclusive but excludes all unrighteousness and sin. Baptism is not a mere welcoming rite but a rite that signifies one's separation from evil. Any theology of judgment has fallen on hard times recently in favor of a softer and gentler message of peace and justice. But with justice comes judgment. It cannot be otherwise.
To ignore judgment leaves the preacher with no reason for preaching the gospel. It is not a matter of scaring people into heaven. It is a matter of revealing the need for salvation and why Jesus is so important. If he is only a common messiah who does what the people expect, then he is no use to us. But if, on the other hand, he is the Messiah who lays bare the pretenses and false expectations of the people and reveals their deep seated need for personal and inner transformation, then he is someone surprising and filled with ultimate and eternal meaning. For preachers to leave out the fire is to let go of the reason for the gospel and thereby cheapen the good news.
Luke is careful to sequence his verbs in verse 21. While Jesus was being baptized and praying, the heavens were opened. Praying is very important in Luke's Gospel. It signifies Jesus' deep devotion to God, the Father, and also signals important events.
The baptism itself signifies Jesus' solidarity with sinners. The descent of the Spirit in bodily form may be a concession to Luke's ancient Gentile readers who would appreciate such a detail after hearing about so many animal omens in their lives. It is clearly not the dove that is important however, but the Spirit's descent upon the "un"-expected one. Jesus' identity is confirmed by this event when the voice from heaven marks him out as not only a man of great worth and note, but as the very offspring of God.
Such titles belonged only to men of great renown such as the emperor Tiberius who had the title "Son of the divine Augustus" placed on his coins. For a commoner from Galilee to receive such a title from such a source is beyond credulity. Yet, as in Mark's Gospel, only Jesus hears the voice. It is not for the people's consumption. The reader is privy to the voice, but not the crowds.
At this point, all the expectations of the people have been left in doubt, but not for the reader. The baptism is Jesus' inauguration in Luke, but it is not an apocalyptic event such as the people would have preferred. One dove hardly signifies the end. Nor is this Messiah someone who merely exacts vengeance upon the enemies of Israel. He is another kind of figure, approved of quietly by heaven, having the power to divide between good and evil, devoted to sinners and to his Father's will.