The promise to Abraham and his descendants, that he would be heir of the world, was not through the law but through the righteousness of faith.

For if those who are of the law are to be the heirs, faith has no value and the promise is void,

Therefore, the promise comes by faith, so that it may be by grace and may be guaranteed to all his descendantsnot only to those who are of the law but also to those who are of the faith of Abraham, for he is the father of us all

He did not waver at the promise of God through unbelief, but was strengthened in his faith and gave glory to God,

In other words, it is not the children of the flesh who are the children of God, but the children of the promise are reckoned as descendants.

For this is what the promise said: "About this time I will return and Sarah shall have a son."