What is the word of truth that brings salvation?
The word of truth is the gospel of your salvation—Paul's gospel that Christ died for our sins, was buried, and rose again. When a sinner hears and believes this gospel, the Holy Spirit of promise comes in and seals them.
Les explains the progression: after hearing the word of truth—the gospel of your salvation—and after believing, the believer is sealed with the Holy Spirit of promise. Every child of God first comes as a sinner, a son of Adam, condemned already; when the Holy Spirit opens understanding and the person believes the gospel, the Spirit comes in as God's brand and power within. This places believers under grace, not law, though grace is never licensed; they are led by the indwelling Holy Spirit rather than the stringent demands of thou shalt and thou shalt not.