Paul thanks God that the Gospel has taken hold among them by the Spirit, in spite of opposition from Jews (chpt 1-2). He is glad to hear they are still thriving, after he worked among them for a time (2-3). Live in purity and love. Control your body and use your hands to provide for yourself. Don't worry about those who have already died, grieving for them as pagans do, without hope. They will be with the Lord first at His return (4). That day will be like a thief in the night for the world. But we are children of day, so it should not surprise us when it comes. In all the ruckus on their first visit (Acts 17:1-9) they apparently still had time to establish elders, to whom Paul points the believers as their leaders (5).