“In hope he believed against hope, that he should become the father of many nations, as he had been told, “So shall your offspring be.”” -Romans 4:18
And here Paul highlights Abraham’s robust faith. Abraham believed God’s promise when all things material were contrary to human reason. He was himself 100 years old and Sarah, 90. They were both beyond child bearing years and what did that matter? God told Abraham that he should become the father of many nations and he believed God rather than that which his reason, or his eyes, or his experience told him. As the author of the letter to the Hebrews notes, Abraham’s was exemplary of the very definition of faith:
“Now faith is the assurance of things hoped for, the conviction of things not seen.” -Hebrews 11:1
“By faith Abraham obeyed when he was called to go out to a place that he was to receive as an inheritance. And he went out, not knowing where he was going. By faith he went to live in the land of promise, as in a foreign land, living in tents with Isaac and Jacob, heirs with him of the same promise. For he was looking forward to the city that has foundations, whose designer and builder is God. By faith Sarah herself received power to conceive, even when she was past the age, since she considered him faithful who had promised. Therefore from one man, and him as good as dead, were born descendants as many as the stars of heaven and as many as the innumerable grains of sand by the seashore.” -Hebrews 11:8–12
Ultimately, Paul is demonstrating that Abraham’s faith—faith that believes what God says regardless of what man can prove— is the kind of faith that saves all men, Jew and Gentile alike.