For this cause, when I could no longer forbear, I sent to know your faith, lest by some means the tempter have tempted you, and our labour be in vain.
For this cause - because I know your "tribulation" has actually begun (1 Thessalonians 3:4).
When I, х kagoo (G2504)] - 'when I also could no longer contain myself.'
I sent. Paul was the actual sender; hence, the "I." Paul, Silas, and Timothy himself had agreed on the mission already, before Paul went to Athens; hence, the "we" (1 Thessalonians 3:1, note).
To know - the state of.
Your faith - whether it stood the trial (Colossians 4:8).
Lest by some means ... have tempted you, and our labour be, х me (G3165) poos (G4459) epeirasen (G3985) kai (G2532) genetai (G1096)] - the indicative in the former sentence, the subjunctive in the latter. 'To know, whether haply (Ellicott, 'lest haply') the tempter have tempted you (the indicative implying such was the case), and lest, (in that case) our labour may prove in vain' (Greek, Galatians 4:11). The temptation was a fact; Paul's fear was, lest haply, by their yielding to it, his labour should prove vain: "vain," so far as ye are concerned; but not as concerns us, so far as we have sincerely laboured (Isaiah 49:4; 1 Corinthians 3:8).