because our gospel did not come unto you in word only, but also in inherent ability and by the Holy Spirit and in much assurance, just as you all know what manner of men we were among you because of you (1 Thessalonians 1:5).
It was their choice to accept the gospel for salvation, which—through abundant assurance and its inherent power to save—produced a transformation in their lives. This stood in stark contrast to many of the Jews, who had rejected the message of the resurrected Messiah.
Paul came to them from Philippi, after having been beaten and imprisoned on false charges. Though his wounds were still fresh, he not only proclaimed the gospel but also openly shared his very life, becoming a father to them.