Rom 4:19-20, 19 He did not weaken in faith when he considered his own body, which was as good as dead (since he was about a hundred years old), or when he considered the barrenness of Sarah’s womb. 20 No unbelief made him waver concerning the promise of God, but he grew strong in his faith as he gave glory to God,
Abraham had no point of reference to strengthen his faith. No one had gone through a situation like his to look to. Though old with also old and barren wife, he still held on to to the promise. Despite his condition getting worse, his faith did not weaken, he paid no attention to those physical obstacles, both in himself and in Sarah, which might seem to render the fulfillment hopeless. He still hanged on to the word though himself as good as dead, he was 100 years old.
Against hope, he believed in hope, no unbelief made him waver in his faith. All the arguments of sense, and reason, and experience, which often support hope, were against him but his confidence in God remained steadfast through it all. Heb 10:35-36 “35 Therefore do not throw away your confidence, which has a great reward. 36 For you have need of endurance, so that when you have done the will of God you may receive what is promised“. 2Co 3:5 “Not that we are sufficient in ourselves to claim anything as coming from us, but our sufficiency is from God”