Summary: If our religion is going to be more than just performing of obligations we have to ask some hard questions.
"There will be signs in the sun, the moon, and the stars, and on the earth distress among nations confused by the roaring of the sea and the waves. People will faint from fear and foreboding of what is coming upon the world, for the powers of the heavens will be shaken. Then they will see ’the Son of Man coming in a cloud’ with power and great glory.
Now when these things begin to take place, stand up and raise your heads, because your redemption is drawing near."
Then he told them a parable: "Look at the fig tree and all the trees; as soon as they sprout leaves you can see for yourselves and know that summer is already near. So also, when you see these things taking place, you know that the kingdom of God is near. Truly I tell you, this generation will not pass away until all things have taken place. Heaven and earth will pass away, but my words will not pass away.
"Be on guard so that your hearts are not weighed down with dissipation and drunkenness and the worries of this life, and that day catch you unexpectedly, like a trap. For it will come upon all who live on the face of the whole earth. Be alert at all times, praying that you may have the strength to escape all these things that will take place, and to stand before the Son of Man.” 
One day in 1982 Larry Walters decided he wanted to see his neighborhood from a new perspective. He went down to the local army surplus store one morning and bought forty-five used weather balloons. That afternoon he strapped himself into a lawn chair, to which several of his friends tied the helium-filled balloons. He took along a six-pack of beer, a peanut-butter-and-jelly sandwich, and a BB gun, figuring he could shoot the balloons one at a time when he was ready to land.
Walters figured the balloons would lift him about 100 feet in the air; he was wrong! The chair soared more than 11,000 feet into the sky – in the middle of the air traffic pattern at Los Angeles International Airport. Too frightened to shoot any of the balloons, he stayed airborne for more than two hours. The airport was forced to shut down its runways for much of the afternoon, causing long delays in flights from across the country.
Soon after he was safely grounded and cited by the police, reporters asked him three questions:
"Were you scared?" "Yes."
"Would you do it again?" "No."
"Why did you do it?" "Because," he said, "sometimes you can’t just sit there, sometimes you gotta do something." 
How different is Larry Walters from the German Pastor Dietrich Bonhoeffer, who during the Second World War opposed Hitler’s Nazis. He was arrested for his opposition to crimes against humanity being conducted by the leaders of his homeland. Shortly before he was executed by hanging at Christmas 1945, Bonhoeffer wrote to his family and friends :
A prison cell, in which one waits, hopes ... and is completely dependent on the fact that the door of freedom has to be opened from the outside, is not a bad picture of Advent. 
Advent is no prison cell, but it is a time of waiting, reflection and anticipation. In the Christian calendar it is the beginning of a new year. Advent is advenire, Latin for “draw near”. We reflect on our closeness to Christ, and anticipate His closeness in coming to us…we reflect on history, and his first coming, we wait in the present day, and we anticipate his coming in the future.
If our faith, our religion is to be more than just fulfillment of duties and obligations, we need to ask the hard questions of Advent. We need to ask the What, Who, When, Why, Where and How questions of this season. Let me try to answer most of that with a single sentence:
We (Christian believers) are to be continually alert, reflecting on the first incarnation of Jesus Christ, and anticipating the second; we do this because Jesus himself told us to keep looking up as our redemption will come in the clouds.
Now, that may handle most of the Advent questions, but, by my count still leaves the “how” unanswered. How shall we keep “looking-up”? How shall we anticipate His coming? The good Doctor Luke wrote down what Jesus said about it in the end of our passage. He tells us to pay particular attention to the culture in which we live, and to be responsive to that which we observe. He told us to read the culture and react wisely to the culture.
READ THE CULTURE
“Be on guard so that your hearts are not weighed down with dissipation and drunkenness and the worries of this life, and that day catch you unexpectedly, like a trap. For it will come upon all who live on the face of the whole earth.”