For this reason, when I could bear it no longer, I sent to find out about your faith, for fear that the tempter had somehow tempted you and that our labor might have been in vain.
Paul sent to learn about their faith, fearing the tempter might have prevailed and the labor proved empty.
It closes the section with the urgency behind Timothy's visit.
1So when we could bear it no longer, we were willing to be left on our own in Athens. 2We sent Timothy, our brother and fellow worker for God in the gospel of Christ, to strengthen and encourage you in your faith, 3so that none of you would be shaken by these trials. For you know that we are destined for this. 4Indeed, when we were with you, we kept warning you that we would suffer persecution; and as you know, it has come to pass. 5For this reason, when I could bear it no longer, I sent to find out about your faith, for fear that the tempter had somehow tempted you and that our labor might have been in vain.