By the word of the Lord: The teachings of Jesus gave Paul assurance for the Thessalonians (see Matthew 24:29–31, 40–41).
will by no means precede: Paul assures the Thessalonians that when the Lord returns, the dead will not be at a disadvantage. They will participate equally in the resurrection.
In talking about the return of Jesus, Paul uses a Greek term (parousia). It is often used to describe an emperor or king's grand arrival in a city (see 1 Thessalonians 4:17).