ectypal.net

...this is confirmed by the fact that Christ has merited eternal life for the elect by subjecting Himself to the law, satisfying it by bearing the punishment of the law and by perfect holiness in both nature and conduct. This is evident in Rom 8:4, where the apostle declared that by virtue of Christ's satisfaction "... the righteousness of the law might be fulfilled in us (the elect)." This is also stated in Gal 4:45: "God sent forth his Son, made of a woman, made under the law, to redeem them that were under the law, that we might receive the adoption of sons." Notice that here reference is made to a law—the same law Adam had. To this law the Lord Jesus subjected Himself, and in doing so He merited redemption and adoption of sons for the elect. "And if children, then heirs; heirs of God, and joint-heirs with Christ ... that we may be also glorified together" (Rom 8:17). Thus, eternal glory necessarily follows upon obedience to the law. Consequently, Adam, having the same law, had the promise of eternal felicity.