Master of the House
There are two ways to interpret this verse. First, Jesus is telling us that because we can never know when He will return, we must therefore be ever vigilant lest we be caught unprepared, as a homeowner who is asleep when the thief breaks in. Or second, that we must be alert because we do not know when He will return, but those who discern and come to know the time of His coming should most certainly be alert and ready for His coming.
I know most of us have been taught the first interpretation and must be wondering where do I come up with the second? From verse 43. If the first interpretation were the true one, there would not be a verse 43. Read the passage without this verse and notice how much clearer and fluid it becomes. By inserting verse 43, I believe Jesus is hinting at a future time when some who gain 'understanding' will discern the time of His return and prepare for it with increased vigor. Like Noah, we will be alerted just prior to the coming Judgment, but only those who are awake in God's Light will see it.
We must remember that this Discourse was given 2,000 years ago, and I doubt if anyone listening at that time would have thought that Christ's return was two millennia away. If Jesus had told them that, how many of them would have been as diligent in their faith. I would guess that at least some would have turned away in disappointment. The anticipation of Christ's return has kept many walking faithfully, but it should be especially true for the generation that begins to see the fig tree 'put forth its leaves'.
I have been anticipating Christ's Second Coming for decades, but I have become more focused in reaching this world for Christ as I see and anticipate the fulfillment of many of the signs pointing to His Return in our lifetime!