This is another section of the Bible that we tend to read in a very self-centered fashion. I can't tell you how many sermons and studies and teachings I've heard about this, inspiring an adrenaline rush of "Jesus is coming back at any moment, and he'll take one person out of bed and leave their roommate!" It's led to songs like Larry Norman's "I Wish We'd All Been Ready" (very catchy) and some frankly abusive teaching that has inspired terror in order to motivate people to true conversion. It's the classic "If you died tonight ..." question that is provocative to think about, but notice -- and I think this is critically important -- that Jesus doesn't ever inspire conversions based on that kind of tactic.
So what's Jesus getting at in this section? Let's make a few observations.
First, remember that "the kingdom of God" is not about some future heaven, but rather it is about the present day rule of God in the lives of believers, looking ahead to a future fulfillment. It's about God's kingship. So verses 20-21 are extremely important. There's an intentional linguistic pun in the text based on the fact that the second person singular and plural are identical in Greek. In other words, Jesus is saying both "the kingdom of God is within you" (singular) AND "the kingdom of God is among you all" (plural). There is an internal, individually spiritual dimension to God's rule. There is also a relational, corporate reality to God's rule. Both are key parts of the kingship of God in our lives.
Then Jesus begins to teach his disciples some difficult things about what life will be like in their future. (Remember we have to try to discern what these words meant to their original hearers before we import them wholesale into our lives.) Jesus and the disciples are traveling toward Jerusalem. At this stage, they are likely on the east side of the Jordan River, traveling the road that followed the floodplain southward from Galilee (crossing along the border of Samaria), then crossing the river and moving southward. They'll re-cross the river near Jericho, as we see in 18:35, and begin the long climb up into the hills where Jerusalem is located.
Interestingly, this puts Jesus and his followers geographically near the place where Lot escaped the destruction of Sodom and Gomorrah. That may be what prompted the conversation Luke records here. Remember, though, that Jesus is looking ahead to two cataclysmic events. First is his own crucifixion, coming up in a few short weeks. Second is that a generation from now, the city of Jerusalem will be destroyed by the Roman armies who will surround it, lay siege to it, and eventually burn the city and the temple. These two events are beyond-comprehension devastating for Jesus' immediate audience. Jesus uses figurative language to communicate how devastating these events will be. First the crucifixion will devastate his followers, throwing their movement into absolute chaos, then (at the time of his resurrection) transforming this movement into something none of them could imagine right now. Then, a generation later, as the news of Jesus' resurrection is spreading outward from Jerusalem into the whole Roman world and beyond, the Jews will rise up in revolt against the Romans and suffer the terrible consequences as Vespasian and his son Titus come with the legions to destroy anyone who would proclaim their independence from Caesar. It's not surprising that Jesus would end this section by saying, in answer to his disciples' question, "Where the corpse is, there the vultures will gather." His own soon-to-be-crucified body and the corpse of burned Jerusalem are both held in this cryptic statement.
Is it realistic to think that Jesus' words here are intended to be more figurative than literal? Think about the way we use language. A decisive political victory is a "landslide." A mass of refugees is a "flood." When public sympathies are changing we say "the tide is turning." An individual or a set of circumstances that seems a bit chaotic and high-energy is a "whirlwind." We do this all the time -- but because these are normal expressions for us, we don't think anything of it. Jesus is doing much the same thing, using common apocalyptic expressions here to help his people understand a bit of what's coming, and to prepare them for the challenging days ahead.
What do we do about applying these things to ourselves? It's certainly not a bad thing to hear these texts as warnings to be ready for whatever cataclysms might come. Jesus has spoken that message over and over again already, especially in chapter 13 where he tells his followers to live in an attitude of repentance. Make peace with God and tend to your human relationships. Be ready. Don't get embroiled in the concerns of daily life and miss the fragile nature of this existence. Watch for what God is doing and be ready to change your plans when God calls you to something new.