Albert Barnes 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)

For this cause. Since I knew that you were so liable to be persecuted, and since I feared that some might be turned from the truth by this opposition.

When I could no longer refrain. (See the comments on 1 Thessalonians 3:1).

I sent to know your faith. That is, your fidelity, or your steadfastness in the gospel.

Lest by some means. Either by allurements to apostasy, set before you by your former heathen friends; or by the arts of false teachers; or by the severity of suffering. Satan has many methods of seducing men from the truth, and Paul was fearful that by some of his arts he might be successful there.

The tempter. Satan; for though the Jews were the immediate actors in those transactions, yet the apostle regarded them as being under the direction of Satan, and as accomplishing his purposes. He was, therefore, the real author of the persecutions which had been excited. He is here called the "Tempter," as he is often (Compare to Matthew 4), and the truths taught are:

1. that Satan is the great author of persecution; and
2. that in a time of persecution—or of trial of any kind—he endeavors to tempt men to swerve from the truth, and to abandon their religion. In persecution, men are tempted to apostatize from God, in order to avoid suffering. In afflictions of other kinds, Satan often tempts the sufferer to murmur and complain; to charge God with harshness, partiality, and severity, and to give vent to expressions that will show that religion has none of its boasted power to support the soul in the day of trial. (Compare to Job 1:9-11.) In all times of affliction, as well as in prosperity, we may be sure that the "Tempter" is not far off, and should be on our guard against his wiles.

And our labour be in vain. By your being turned from the faith. (See the comments on Galatians 4:11).