Jamieson, Fausset & Brown

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.