Jamieson, Fausset and Brown on Romans 4:13-15

13-15. For the promise, &c.--This is merely an enlargement of the foregoing reasoning, applying to the law what had just been said of circumcision.

that he should be the heir of the world--or, that "all the families of the earth should be blessed in him."

was not to Abraham or to his seed through the law--in virtue of obedience to the law.

but through the righteousness of faith--in virtue of his simple faith in the divine promises.