Summary: A familiar text with many preaching paths. Once again we need the wilderness, the familiar cry of John the Baptist, to restore our sights. To put it another way, Advent remains a reason of repentance (whatever color we now use), but one where repentance isn't simply about personal sins, but a reorientation of our whole mind away from the crap out there about Christmas and toward the salvation of God unfolding in Jesus Christ.
τετρααρχουντες ("rule as tetra-arch"; 3:1) The word tetra-arch means rule as a piddly regional governor. Luke includes a number of historical details in his Gospel, especially early on; Luke clearly wants to show that Jesus birth and life are actual events.
βαπτισμα ("baptism"; 3:3). Originally, this word did not have religious meaning. It simply meant to dip. For your enjoyment, here are the Liddell-Scott Hellenistic meanings of the word. Wow!
I. trans. to dip in water
2. to dip in poison
3. to dip in dye, to dye
4. to draw water
II. intransitive the ship dipped, ie, sank
Try preaching that: Baptism as a dip in poison; as a dip in dye; as a drawing of water from God; as finally, a sinking ship!
***Warning on the next word: The first four sentences are stuff you all know. But read on...
μετανοεω ("repent"; 3:3) The Greek meaning of the word is "new mind." Stories later in the Gospel -- Bartimaus or the woman anointing Jesus -- show someone whose life is transformed by Jesus. So it may not be explicit (in that the word repentance may not be used), but the repentance continues. In Lidell-Scott's ancient (and secular) Greek lexicon, repent means to change one's mind or purpose. We often put repentance together with sin, a fine thing, but perhaps we need to consider that repentance means often more than simply a struggle against temptation, but a paradigm shift, a transformation of our whole outlook, if not way of life and even being. In this case, there is a shift into the forgiveness of sins. Perhaps that is what Jesus ministry is really about, not simply our own forgiveness, but inculcating a world view that finally includes forgiveness. Perhaps this is σωτηριον (salvation): when the world finally embraces forgiveness as the path. Overarching point: μετανοεω in Greek and in the New Testament means far more than forgiveness of sins. (Or forgiveness of sins means far more than we think it does).
πληρωθησται (πληροω, fill or fulfill, 3:5) and ταπεινωθησται (ταπεινω, fulfill, 3:5): The English renders these words as "raised up" and "made low." Yet Luke (and Isaiah) use the words here for fill and humble. These then echo other parts of Luke's Gospel (the Magnificat; Jesus words on the road to Emmaus). These represent key features of Jesus mission: To fulfill and to humble.
Grammar note: Lack of punctation in ancient languages
Original Hebrew and Greek manuscripts lack punctuation; it was added later by monks. So we really don't know if Isaiah meant, "A voice cries out, 'In the wilderness prepare the way of the Lord'" or "A voice cries out in the Wilderness, 'prepare the way of the Lord'." The monks thought the former...probably good to go with their instinct.
In that case, it seems that the Gospel writers change the punctuation to fit their own program of matching John's work with the description in Isaiah.
A few options: The scholarly one: Preach or teach, in a despising fashion, about how the NT abuses the OT
The Christological one: Preach and teach about how the NT rightfully abuses the OT to make it fit with Christ!
Or the pastoral one: Say, in this case, both are valid. John the Baptist cries out in the wilderness. Yet he speaks to each of us to get into the wilderness, away from all the chaos of the world, to focus on God and God alone.