"Christ brought in a new covenant which continued the sign of the 
Sabbath, and prepared another people, by writing his law upon their hearts.
These now are the true Israel; for the changing of the subjects never did, nor ever can change the moral law of God. Therefore Paul argues the circumcision of the heart, and says that they are not all Israel which are of Israel, neither because they are the seed of Abraham, are they all children; but, in Isaac shall thy seed be called; That is, they which are they children of the flesh, these are not the children of God; but the children of the promise are counted for the seed. Romans 9:7,8
These now are the true Israel; for the changing of the subjects never did, nor ever can change the moral law of God. Therefore Paul argues the circumcision of the heart, and says that they are not all Israel which are of Israel, neither because they are the seed of Abraham, are they all children; but, in Isaac shall thy seed be called; That is, they which are they children of the flesh, these are not the children of God; but the children of the promise are counted for the seed. Romans 9:7,8
Now if the children of God are the true Israel, and if the Sabbath was given as a sign forever, and a perpetual covenant, I ask, how can it be abolished while there is one Israelite remaining to claim the promise?
You have evidently noticed, that all the difficulties on the Sabbath question among Christians have arisen from the foolish notion, that Israel meant only the literal Jew. But when we understand Israel to mean the people of God, the difficulties, every man must acknowledge, all vanish at once." T.M. Preble