For this cause we also thank God without ceasing, that, when you received from us the word of the message of God, you accepted it not as the word of men, but, as it is in truth, the word of God, which also works in you who believe.
which effectually worketh also in you that believe: The gospel is the power of God unto salvation (Romans 1:16). Like those prophetic utterances of old, "It shall not return unto me void" (Isaiah 55:11). The frequency with which this verb ("worketh") and its cognates refer in the New Testament to the activity of God makes it all but certain the reference here is to God’s work in the lives of believers. Lending support to that notion is the fact that some scholars believe that, in the Greek, the verb is passive rather than active. The word "is at work in you" (NIV). As Morris notes, "the power is God’s and the word is his instrument" (63). "The message came from God through the apostle to the Thessalonians and was changing them" (Stott 54).