Galatians 3:11

But that no one is justified by the law in the sight of God is evident, for “the just shall live by faith.”

Romans 4:19-21 (NCV)

…Abraham thought about all this, but his faith in God did not become weak. He never doubted that God would keep his promise, and he never stopped believing. He grew stronger in his faith and gave praise to God. Abraham felt sure that God was able to do what he had promised.