Look at that again. God's grace guarantees God's promise - namely, the promise that we will be heirs of the world.
Grace guarantees the promise in two ways.
It overrides our demerit. We come to God as sinners. And God overrides our demerit and counts us as righteous for the sake of Christ. What Paul has in mind is the birth of Isaac when Abraham was almost 100 years old and Sarah his wife was 90 and barren. Isaac was the son of promise, the one through whom the promise would be fulfilled.