1 Thessalonians 3:5 reads, “So when I could bear it no longer, I sent to find out about your faith, for fear that the tempter somehow tempted you and our toil had proven useless. The thought expressed in verse five is that Paul was afraid that somehow or perhaps something bad might happen as a result of the tempter enticing them to sin, and not just sin but abandon the gospel. He is the cause of the temptation to sin, not God.
The second part of the verse implies the fear that if Satan is successful in causing them to sin, his work would prove useless. Sometimes, depending on the construction of the sentence, the conjunction “and” is a marker of result which means that it is introducing a statement that represents the result of the previous one expressing Paul’s concern that Satan caused them to be tempted to sin. The conjunction kai is a marker of result which means that it is introducing a statement that presents the result of the previous one which expresses Paul’s concern that Satan caused the Thessalonians to be tempted to sin.
The labor that he feared would be in vain implies intensive work with great difficulty. He and Silvanus experienced hardships and persecution as a result of their preaching and to not receive the benefit of such labor would be useless. In other words, it would be worth all the pain and persecution if the Thessalonians remained true to God. But, if they abandon God, such labor would be useless. Literally, the word useless means empty or without content as opposed to being full. The word was used of letting people leave your house empty-handed. Figuratively, it is used of persons being hollow, vain, shallow, careless, lacking in judgment (tou nou kenos, empty in mind, and also in the ethical sense of ineffectual or vain. When the word is used metaphorically or figuratively, it means lacking content (e.g. “empty words” or missing effect, thus vain). The adjective form, also, is used to speak of things being without basis, truth, or power (“empty words”, Eph. 5:6, Col. 2:8). Here, however, it is used metaphorically in the sense of work, endeavors, or acts that result in nothing, without profit, of no effect or without reaching its goal (Acts 4:25; 1 Cor. 15:10, 14, 58; 2 Cor. 6:1; Gal. 2:2; Phil. 2:16; 1 Thess. 2:1; 3:5; Jas. 2:20).
In 1 Thessalonians 3:5, the adjective kenos means “without result” since it pertains to lacking in results. The word is the object of the preposition eis, which is a marker of a change of state. Therefore, this prepositional phrase is expressing the idea of Paul and Silvanus’ labor among the Thessalonian Christian community without result which it was not before giving into Satan’s temptation to sin.