Now to the one who works, his pay is not credited due to grace but due to obligation.

So even David himself speaks regarding the blessedness of the man to whom God credits righteousness apart from works:

But to the one who does not work, but believes in the one who declares the ungodly righteous, his faith is credited as righteousness.

This means it is not the children of the flesh who are the children of God; rather, the children of promise are counted as descendants.