By the time Luke wrote this gospel, the followers of Jesus would have experienced the destruction of the temple (see Luke 21:5-6) and of the city of Jerusalem (Luke 21:20-24). He ties together disruptions caused by humans and by nature to a promise of God's action. These words of Jesus would guide them to a response to the turmoil of their times, and us in ours.
We can be as sure of God's intervention in upsetting times as we can be of a tree's leaves indicating that summer is coming.
Notice that everybody will experience that summer. Not just especially religious people. Not just especially religious people that also go to our church. Summer comes for everyone--whether they deserve better weather or not.