What does 1 Thessalonians 3:13 mean?

Paul strongly desired that God would establish the Thessalonian believers' hearts blameless in holiness at the return of Jesus with all His saints. Paul did not pray that they would be sinless but blameless—free of all valid accusations. No one can attain sinlessness in this life. The apostle John said, "If we say we have not sinned, we make [God] a liar, and his word is not in us" (1 John 1:10). Even the apostle Paul, who led a holy life, recognized that he was a sinner. In 1 Timothy 1:15, after writing that Christ Jesus came into the world to save sinners, Paul added, "of whom I am the foremost." He did not say he had been a sinner. He said, "I am the foremost sinner." All human beings are sinners. They are either lost sinners or saved sinners.

Who are the saints that will return with Jesus? They must be departed Christians, whose bodies are in the grave. When they return with Jesus, they will receive resurrected bodies. Paul discusses this event in the next chapter.