Note that the word of God was not at work in the lives of all who lived in Thessalonica (not even all those who heard the gospel), but only in the lives of those who believe. Though the gospel is the power of God for salvation, that power is only experienced by those who believe (Romans 1:16–17). This final word pair is critically important for understanding how a person may experience and receive the power of God through his word.1
13 And we also thank God constantly for this, that when you received the word of God, which you heard from us, you accepted it not as the word of men but as what it really is, the word of God, which is at work in you believers.