Romans 4:13

Romans 4:13. For the promise, that he should be the heir of the world, was not to Abraham, or to his seed, through the law, but through the righteousness of faith.

The law was promulgated on mount Sinai four hundred years after the covenant of grace was made with Abraham the father of believers, and so made with all believers, for they are his true seed, and God has entered into a covenant of grace and salvation with them.