What does 1 Thessalonians 3:5 mean?
Paul had sent Timothy to Thessalonica to support the Christians there (1 Thessalonians 3:1–2), but also to find out whether or not the believers' faith was withstanding persecution. He calls the Devil "the tempter," knowing that he had tempted Eve in the garden of Eden, numerous believers through the ages, and even the Lord Jesus Himself (Matthew 4:1–11). Of course, Jesus successfully withstood the Devil's temptations and is available to help believers stand against the Devil (Hebrews 2:18).Paul was concerned that the faith of the believers at Thessalonica might waver and weaken under the Devil's temptation, which would stunt their spiritual growth. In that case, Paul's labor on their behalf to produce spiritual fruit in their lives would be curtailed. Hebrews 4:14–16 encourages believers to pray confidently when we are feeling weak. We should realize that Jesus experienced trials like ours and successfully overcame them. As our great High Priest, He is able to provide help when we need it.
The following passage contains Paul's delighted reaction at learning that these Christians had, indeed, been able to "hold fast" to their faith (Hebrews 3:6).