Reading through Romans

"Therefore it is of faith that it might be according to grace, so that the promise might be sure to all the seed, not only to those who are of the law, but also to those who are of the faith of Abraham, who is the father of us all"

 It is of faith that it might be according to grace: Faith is related to grace in the same way works is related to law. Grace and law are the principles, and faith and works are the means by which we pursue those principles for our relationship with God.

 To speak technically, we are not saved by faith. We are saved by God's grace, and grace is appropriated by faith

It is of faith: Salvation is of faith, and nothing else. We can only receive salvation by the principle of grace through faith.Grace cannot be obtained through works, whether they be past, present, or promised because by definition grace is given without regard to anything in the one who receives it.

"Grace and faith are congruous, and will draw together in the same chariot, but grace and merit are contrary the one to the other and pull opposite ways, and therefore God has not chosen to yoke them together. He will not build with incongruous materials, or daub with untempered mortar. He will not make an image partly of gold and partly of clay, nor weave a linsey-woolsey garment: his work is all of a piece and all of grace." (Spurgeon)

So that the promise might be sure to all the seed: The promise can only be sure if it is according to grace. If law was the basis of our salvation, then our salvation would depend on our performance in keeping the law - and no one can keep the law good enough to be saved by it. A law-promise of salvation can never be sure.

 If the promise "were of the law, it would be unsure and uncertain, because of man's weakness, who is not able to perform it." (Poole)

But also to those who are of the faith of Abraham, who is the father of us all: If our relationship with God is according to grace (not circumcision or law-keeping), then that relationship is for those who are of the faith of Abraham, even if they are not of his lineage.

A Gentile could say, "I am not a Jew, I am not of the law; but I am of the faith of Abraham," and he would be just as saved as a Jewish believer in Jesus would be.

The father of us all: The fulfillment of the promise inGenesis 17:4-5 is found not only in Abraham's descendants through Isaac, but especially in his role as being the father of us all who believe - and those believers come from every nation under heaven.