Here is an excerpt from an upcoming Study on 1 Thessalonians. I lean Epexegetical (Explanatory).
4 We know your choosing, brothers and sisters loved by God, 5 because our Good News did not come to you in word only, but also with power, that is, with the Holy Spirit and [with] full conviction, just as you know what kind of men we became [among] you for your sake.
But that may not be all that is going on with this passage. At the beginning of verse 5, ὅτι (because) introduces the clause. The way that the passage in translated above, uses ὅτι as the “cause” that Paul knows that God has chosen the Church in Thessalonica, but that is but one way to translate ὅτι. ὅτι can also be translated as “that”, thus the passage would now be epexegetical or explanatory, thus explaining the way the Church in Thessalonica was chosen (Fee, NINTC, 31-32) (Fee, GEP, 40) (Green, PNCT, 93-94). If this is the case, then verses 4 and 5 may be translated: “We know your choosing, brothers and sisters loved by God, that our Good News did not come to you with message only, but also with power, that is with the Holy Spirit and [with] complete certainty...” If this analysis is correct, then we are seeing a conversion experience described here. The missionaries’ message went forth, accompanied with power (miracles?) which is brought on as the Thessalonians are filled with the Holy Spirit which leads to the full certainty that the Thessalonians had truly been converted. In the end, the meaning here is probably two-fold, The message (word) of the Gospel (Good News) in which the Missionaries preached was Spirit empowered and lead to the conversion of the people in the Thessalonian Church (Fee, NINTC, 36).