One of the reasons that Paul expressed thanks to God for the Thessalonians was that he knew God loved them and had chosen them for salvation (1 Thess 1:2, 4). Paul based his knowledge of God’s love and choice on their exemplary conversion (cf. 1 Thess 1:6–10). He also based this knowledge on how the gospel came to them. He knew God chose them “because our gospel came to you not only in word, but also in power and in the Holy Spirit and with full conviction” (1 Thess 1:5). 

Exploring some points of how the gospel came to them, we are encouraged along the way. 

First, the gospel came “in word,” which was absolutely necessary. How else would they know the gospel but through the words of a gospel preacher (cf. Rom 10:14)? 

Second, by connecting the next phrases with “not only… but also,” Paul emphasized how the gospel came through him, Silvanus, and Timothy—“in power,” “in the Holy Spirit,” and “with full conviction.” In other words, it was not the Thessalonians who are described in 1 Thess 1:5 (though it is certainly true that they experienced God’s power, Spirit, and conviction as they heard the gospel; cf. John 16:8–11). Rather, it was Paul, Silvanus, and Timothy who are described in this verse. Just as they gave the gospel in word, so also their word was accompanied with power, the Spirit, and conviction. These phrases describe their preaching.

Third, this “power” was maybe seen in miracles just as Paul tells the Romans of “the power of signs and wonders,” made possible “by the power of the Spirit of God” (Rom 15:19). If so, Paul tells us something that Luke did not in Acts 17:1–9. Another option, however, is to understand this “power” in terms of God’s supernatural work in in their preaching. This power came from the Holy Spirit who granted them “full conviction,” a Spirit-given certainty that their gospel words were unassailably true. (Again, this is something for those who believe as well—cf. Col 2:2.) 

Fourth, if we were still wondering who is in view in 1 Thess 1:5, Paul ends the verse by describing his company one more time: “You know what kind of men we proved to be among you for your sake.” They there the kind of men whose word was described in terms of power, the Spirit, and conviction. 

Like the Thessalonians, we are assured of God’s love and choice of us when we faithfully live the Christian life (cf. 2 Pet 1:10–11 with 1 Thess 1:4, 6–10). And, as we just saw, we are assured further when we recount how a messenger gave us the gospel in power, the Spirit, and conviction. The gospel can do a saving work in spite of an unworthy preacher (cf. Phil 1:15–18), but it can be all the more effective when preached with power, the Spirit, and conviction.