For the promise to Abraham and to his seed that he should be heir of the world wasn't through the law, but through the righteousness of faith.

For if those who are of the law are heirs, faith is made void, and the promise is made of no effect.

For this cause it is of faith, that it may be according to grace, to the end that the promise may be sure to all the seed, not to that only which is of the law, but to that also which is of the faith of Abraham, who is the father of us all.

Yet, looking to the promise of God, he didn't waver through unbelief, but grew strong through faith, giving glory to God,

That is, it is not the children of the flesh who are children of God, but the children of the promise are counted as a seed.

For this is a word of promise, "At the appointed time I will come, and Sarah will have a son."