Romans 4:11-13, 16

and he received the sign of circumcision, a seal of the righteousness of the faith which he had while he was in uncircumcision: that he might be the father of all them that believe, though they be in uncircumcision, that righteousness might be reckoned unto them; and the father of circumcision to them who not only are of the circumcision, but who also walk in the steps of that faith of our father Abraham which he had in uncircumcision. For not through the law was the promise to Abraham or to his seed that he should be heir of the world, but through the righteousness of faith.

For this cause it is of faith, that it may be according to grace; to the end that the promise may be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham, who is the father of us all