Paul concluded this epistle with an emphasis on unity in the church to motivate his readers to work out their problems and reestablish peaceful conditions that would glorify God.
3:16 He concluded with two more prayers, his fourth and fifth (v. 18) in this epistle (cf. 1:11-12; 2:16-17; 3:5). He knew that without the Lord's convicting work his instructions and exhortations would be ineffective. His main concern was for peace in the church that could only take place as all the Christians obeyed the truth. God is the source of peace that a church enjoys to the extent that all of its members relate submissively to the will of God. Peace is possible even in the midst of persecution (cf. John 16:33).
3:17 In view of the letter claiming to have been Paul's that the Thessalonians had received (2:2), the apostle felt it necessary to prove that the present one really came from him. He added a word of greeting in his own hand, as he usually did, to authenticate his epistles for the benefit of recipients (cf. Gal. 6:11; 1 Cor. 16:21; Col. 4:18). An amanuensis evidently penned the rest of the letter (cf. Rom. 16:22).
"It was no uncommon thing in ancient letter-writing for the sender, having dictated the bulk of the letter, to write the last sentence or two in his own hand. This is the best explanation of the change of script at the end of several papyrus letters which have been preserved. This practice would help to authenticate the letter (for readers who recognized the sender's writing); a more general purpose would be to make the letter look more personal than one written entirely by an amanuensis."77
3:18 The final benediction is the same as the one that ends 1 Thessalonians except for the addition of the word "all"here.
"If any theological point is to be made from the inclusion of all,' it is perhaps that Paul asked for Christ's grace even on those who were not holding to the Christian pattern of behavior regarding work."78
Paul's concern for the peace and unity of all the church was his great passion in this epistle.
