What does 2 Thessalonians 3:18 mean?

In this verse Paul pronounces a benediction. He invokes the grace of our Lord Jesus Christ for all the Thessalonian believers. His use of the pronoun "our" emphasizes the personal relationship he and the Thessalonians had with Jesus. They belonged to him, but in a real sense he also belonged to them. Faith had joined them together eternally.

Divine grace is God's unmerited favor. By grace God saves us. Ephesians 2:8 states plainly, "For by grace you have been saved." Grace enables us to withstand trials. When Paul was undergoing a difficult trial or malady, he prayed that the Lord would remove what he called his thorn in the flesh, but the Lord did not remove it. However, He assured Paul that His grace was sufficient for him (2 Corinthians 12:8–9).

That same all-sufficient grace is available to all who belong to the Lord Jesus Christ. Second Timothy 2:1 teaches us that divine grace strengthens us. Titus 2:11–13 credits God's grace with "training us to renounce ungodliness and worldly passions, and to live self-controlled, upright, and godly lives in the present age, waiting for our blessed hope, the appearing of the glory of our great God and Savior Jesus Christ." So Paul's benediction was a fitting conclusion to his letter.