John Calvin Commentary 1 Thessalonians 1:5

"how that our gospel came not unto you in word only, but also in power, and in the Holy Spirit, and [in] much assurance; even as ye know what manner of men we showed ourselves toward you for your sake." — 1 Thessalonians 1:5 (ASV)

As you know. Paul, as I have said before, has as his aim that the Thessalonians, influenced by the same considerations, might entertain no doubt that they were elected by God. For it had been God's design, in honoring Paul’s ministry, to manifest their adoption to them. Accordingly, having said that they know what manner of persons they had been, he immediately adds that he was such for their sake, by which he means that all this had been given to them, so that they might be fully persuaded that they were loved by God, and that their election was beyond all controversy.