Notice the way Paul says it in verse 16. Because God's grace is what gives the guarantee. The only way that our eternal future can be guaranteed is if it rests on God's grace. Grace is the free and undeserved work of God to bring his people to glory. Grace is the mighty, omnipotent purpose of God to make sure we get our inheritance.
Grace is the ground of our guarantee.
Now I want you to taste the glory of this powerful promise-guaranteeing grace. To do that, let's look back to something we have seen and then forward to something we have not seen. But to the one who does not work, but believes in Him who justifies the ungodly, his faith is credited as righteousness. Paul is talking here about how to be justified, how to have a righteousness before God that will inherit the promise. Notice the opposite of grace and faith in these verses.