Paul blesses his readers with words of goodwill from God, that the Thessalonians would experience His peace and favor always.
Paul concludes his letter to the Thessalonians, Now may the Lord of peace Himself continually grant you peace in every circumstance. The Lord be with you all! Though they face much persecution and suffering, Paul prays for peace to come to them in every circumstance from God, the Lord of peace Himself, the only one who can provide spiritual peace even in the worst of situations (Philippians 4:7). In wishing the Thessalonians peace, Paul likely has in mind the Jewish concept of “shalom” which is a broad concept that reflects harmony in every area of life. To have peace in every circumstance would mean that their spiritual perspective would supersede every material circumstance such that it brought a harmony even to material difficulties. The Thessalonians would, through the spiritual perspectives they chose, be living above the circumstances of life.
Paul signs the letter with his own signature, I, Paul, write this greeting with my own hand, and this is a distinguishing mark in every letter; this is the way I write. It is speculated that he had poor eyesight, which is why he did not write his letters himself. But it may be that whoever wrote this letter per his dictation (Timothy, perhaps) was simply a more skillful scribe, and here Paul is signing his own name, his distinguishing mark just as a king seals an official decree with his special seal. It is proof that Paul authored the letter, even if the rest of it was not in his handwriting.
He ends by blessing the Thessalonians as he often does in his epistles: The grace of our Lord Jesus Christ be with you all.