What happens the instant someone believes the gospel?
The moment someone believes the gospel, the Holy Spirit comes in and seals them, becoming God's brand and the very power of God within. They are sealed with the Holy Spirit of promise, transitioning from condemned sinner to child of God under grace, not law.
Les explains the order of salvation: every child of God first comes from being a sinner, a son of Adam, condemned already. As soon as we realize we are sinners, we hear the gospel—the word of truth, the gospel of our salvation—and the Holy Spirit opens our understanding so we believe. Paul's gospel is that Christ died for our sins, was buried, and rose again. The moment we believe, the Holy Spirit comes in, becomes God's brand upon us, and becomes the very power of God within us. This is being sealed with the Holy Spirit of promise. Les notes this is why we are not under law but under grace, though grace is never a license to be lawless; we are now led and guided by the indwelling Holy Spirit.