The law hadn't been given yet. And the law which came later is unable to void the promise that came first. It is only because of the promise that you can have any assurance for your salvation. If the promise was not by faith, then there would be no certainty. If God's promises depend upon my obedience, then I cannot be certain of anything.
Faith leads to Family.
The scope of salvation through faith went far beyond the Jews. And because it did, there is a sense in which the family of Abraham goes far beyond mere physical offspring to encompass all who believe. This is in contrast to the Law. That means the promises of the older testament are ours to claim. God doesn t have one program for Jewish people and another for the church.