“Therefore, the promise comes by faith, so that it may be by grace and may be guaranteed to all Abraham’s offspring — not only to those who are of the law but also to those who have the faith of Abraham...”Romans 4:16
Millions of Jewish people are the descendants of Abraham. All of his children, grandchildren, great-grandchildren, great-great-grandchildren and so on make up his physical family. But it’s even broader than that! Any Christian who has the faith of Abraham is part of the family too. The promise, part of which says Abraham will inherit the world, comes by faith rather than by physical descent, so it can be guaranteed to all of his descendants.
Righteousness is one of the by-products of believing God. Abraham’s faith was credited to him as righteousness, and amazingly, the same is true for us. Religious observance such as circumcision doesn’t make us right with God. Abraham was declared righteous even before he was circumcised. Your faithful Bible reading, prayer, church attendance, feeding the hungry, giving to the poor, fasting, etc., are good things. But none of these makes you righteous in God’s sight. God is the Father of us all simply because He has set His sight on us, not because of anything we have done.
A child belongs to his father not because the child has been obedient. It’s because he’s part of the family, and that cannot be undone.