(The following is taken from DAVID GUZIK COMMENTARY ON THE BIBLE, Studylight.org)
But the end of all things is at hand; therefore be serious and watchful in your prayers.
The end of all things is at hand: If we really believe that we live in the last days, it is all the more appropriate that we give ourselves to prayer (therefore be serious and watchful in your prayers).
Many Christians who believe that Jesus is coming soon based on prophecy charts and political events fail to apply that belief the proper way. They fail to applying themselves to more diligent prayer.
Therefore be serious . . . in your prayers: We must give ourselves to serious prayer. As we see the weight of eternity rushing towards us, we dare not take the need for prayer lightly.
Therefore be . . . watchful in your prayers: We must give ourselves to watchful prayer, primarily having our hearts and minds watching and ready for the return of Jesus Christ. But this also means watching ourselves and watching this world, measuring our readiness for Jesus’ coming.