John Calvin Commentary 1 Thessalonians 3:5

"For this cause I also, when I could no longer forbear, sent that I might know your faith, lest by any means the tempter had tempted you, and our labor should be in vain." — 1 Thessalonians 3:5 (ASV)

Lest perhaps the tempter has tempted you. By this statement, he teaches us that temptations are always to be feared, because it is the specific work of Satan to tempt. However, since he never ceases to set ambushes for us on all sides and to lay snares for us all around, we must be on our watch, taking careful heed.

He now says openly what he had initially avoided saying because it was too harsh — namely, that he had been concerned that his labors might be in vain if, perhaps, Satan should prevail. He does this so that they may be carefully on their guard and may rouse themselves more vigorously to resist.