To understand the force of Paul’s words here, consider the situation described in Acts 17:2–5. In that passage, 1 Thessalonians 1:2–3 describe Paul’s preaching of the gospel in Thessalonica and 1 Thessalonians 1:4–5 show that some responded in faith while others rejected the message. The preaching, as far as Paul was concerned, was the same in both cases. If 1 Thessalonians 1:5a refers to the power inherent in the preaching, then the logic of the passage is something like this: We know that God has chosen you because our preaching in your midst was powerful.
But that would imply that everyone who heard the powerful preaching was chosen by God, including those who rejected the message. While it is undoubtedly true that Paul’s preaching was characterized by the Spirit’s power, that is not the focus of his attention here; rather, the apostle draws attention to the power at work in the Thessalonian believers—that power which called them inwardly to faith in Christ, and which therefore distinguished them from their unbelieving compatriots. Paul [is] referring to the power from the Holy Spirit that causes the spoken word to penetrate the hearts and minds of its hearers...[the] gospel message was not only a heard word but also an experienced power.
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5 because our gospel came to you not only in word, but also in power and in the Holy Spirit and with full conviction. You know what kind of men we proved to be among you for your sake.