“…and not all are children of Abraham because they are his offspring, but ‘Through Isaac shall your offspring be named.’ This means that it is not the children of the flesh who are the children of God, but the children of the promise are counted as offspring.” (Romans 9.7-8 ESV)
Scripture speaks of the children of Abraham as children of promise. The Jewish people often spoke in such a manner. Here Paul points out that not all children of Abraham were children of promise. Only those who were born through the line of Isaac were those who were heirs of the promise. He was a son of Abraham along with Ishmael. Ishmael was born to Abraham and Hagar and was not an heir of any promise. Later, Isaac had two children with his wife Rebekah: Esau and Jacob. Before they were born God said the older (Esau) would serve the younger (Jacob). Thus, not even all of Isaac’s line would be heirs to the promise.
In time, Jesus came and those not of Isaac’s line were made worthy to become adopted into his line by Jesus’ sacrifice on the cross. Those who receive Jesus’ sacrifice by having faith in Him become children of the promise, not by flesh but by adoption.
We are not saved by keeping the law (though, those who are saved will naturally begin to honor the law, to do what is right, by displaying the fruit of the Spirit [Galatians 5.22-23]). We are not saved by inheriting salvation. We are not saved by works we can perform. We are only saved by surrendering our lives to Christ, having faith in Him. That goes for all people- even those who are of the lineage of Isaac.