The outcome which Paul was at pains to prevent was that their efforts might have been useless. The ESV translates it well according to the more literal meaning: Paul was afraid that his labour (Greek: κόπος) would be in vain (Greek: εἰς κενὸν). There is a close verbal parallel between Paul’s words here and the Septuagint translation of Isaiah 49:4. The Isaiah passage reflects the words of God’s chosen servant, and Paul’s use of them shows that he views his own apostolic calling as part of the biblical prophetic tradition.1 The Greek word κενός
can mean false, deceptive (as in 1 Thessalonians 2:1), but with εἰς it means futile, in vain (2 Corinthians 6:1; Galatians 2:2; Philippians 2:16).2 Paul’s efforts would have been in vain if some or all of the Thessalonian Christians were to abandon their faith in Christ.
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.