Paul’s concern here was that Satan might have tempted the vulnerable new believers to abandon their faith. This was part of a wider and apparently more coordinated spiritual opposition to the work of God, since Satan had also prevented Paul from returning to Thessalonica (1 Thessalonians 2:18). Paul’s letters show that he was profoundly aware of the work of Satan in opposing God’s work and tempting believers to sin (1 Corinthians 7:5; 2 Corinthians 2:11; Ephesians 4:27; Ephesians 6:12; 1 Timothy 3:7; 1 Timothy 5:15; 2 Timothy 2:26), and that is why he urges the Ephesians to stand firm against the devil in the strength of the Lord (Ephesians 6:10–11). In the case of the Thessalonians, this spiritual opposition was expressed through the persecution and hardships that they were suffering. Paul knew that to withstand this pressure they needed to be strengthened in their faith.
5 For this reason, when I could bear it no longer, I sent to learn about your faith, for fear that somehow the tempter had tempted you and our labor would be in vain.