In 1 Peter 4:7 it says, “The end of all things is at hand; therefore be serious and watchful in your prayers. …” Here we see a good reason for vigilance in our prayer: because the end of all things is at hand—and He is coming!
There are also three other passages, in the gospels, which admonish us to be alert in preparation for His coming: in Matthew 24:42-44, in Mark 13:33-37, and in Luke 21:34-36. These gospel passages are written primarily to those living during the tribulation period, to both non-believers (to be saved and ready to enter into His kingdom), and to believers (to have strength to continue trusting Him and praying for strength to escape all the horrible things that are coming upon them, Lu. 21:36). However, the application here to alertness no doubt extends to us (the church) as well, to be ready for His return for us in the rapture.
1 Thessalonians 5:4-11 is clearly addressed to the church. Here we are admonished to be alert so that we will be ready for Him when He snatches us up (1 Thess. 4:17, 1 Cor. 15:51-52)—if we are still alive when He comes.
If you are a Christian, that day should not overtake you like a thief, because you are already saved. Nevertheless, you still need to be alert, not so much to be ready just at that single day when He comes, but so that you can pray without ceasing (the NASB says, “for the purpose of prayer”). And why is this important? Well prayer is always important; but in the light of His return, it is important so that when He returns He will have a reason to be proud of us—for I believe that our faithfulness here will greatly have an effect on our next life there. Yes, everything we do now, whether good or bad, will affect our relationship with Jesus for eternity.