We have this problem sometimes where I work: people sign a contract agreeing to pay us a certain fee, we do the work, and then they change their minds. But by the time they’ve signed a contract, and we’ve done the work, it’s too late to re-negotiate how much they’re willing to pay us.
Paul uses this concept to illustrate the point of v17-18.
We have a covenant with God. It’s not a new covenant. It’s the same one that God made with Abraham. When God gave Israel the law 430 years later, he wasn’t changing the terms of the covenant. Our salvation is still on the same terms as God’s covenant with Abraham. Our inheritance comes by God’s promise, and he promised to save us through Christ, so don’t let anyone tell you “you’re saved by grace but now you need to do something for him in return”. The covenant doesn’t say that, so it’s not true.
Similarly to the way Paul talked about nullifying God’s grace at the end of Chapter 2, if we make up things for us to do to secure our salvation, we’re making God’s promise to save us through Christ “void”, or worthless. Don’t insult him like that. Believe him when he says your salvation is totally secure in Christ. Don’t say “Yeah, I believe you, but I just want to do this thing myself to make extra sure.”