He grew strong in his faith as he gave glory to God, fully convinced that God was able to do what he had promised. ( Romans 4:20 20 yet, looking unto the promise of God, he wavered not through unbelief, but waxed strong through faith, giving glory to God, –21)

Paul has in mind a special reason why faith glorifies God’s future grace. Simply put, the reason is that this God-glorifying faith is a future-oriented confidence in God’s integrity and power and wisdom to follow through on all his promises.

Paul illustrates this faith with Abraham’s response to the promise of God: that he would be the father of many nations ( Romans 4:18 18 Who in hope believed against hope, to the end that he might become a father of many nations, according to that which had been spoken, So shall thy seed be. ). “In hope he believed against hope,” that is, he had faith in the future grace of God’s promise.

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. No unbelief made him waver concerning the promise of God, but he grew strong in his faith as he gave glory to God, fully convinced that God was able to do what he had promised. ( Romans 4:19 19 And without being weakened in faith he considered his own body now as good as dead (he being about a hundred years old), and the deadness of Sarah's womb; –21)

The faith of Abraham was a faith in the promise of God to make him the father of many nations. This faith glorified God because it called attention to all the resources of God that would be required to fulfill it.

Abraham was too old to have children, and Sarah was barren. Not only that: How do you turn a son or two into “many nations,” which God said Abraham would be the father of? It all seemed totally impossible.

Therefore Abraham’s faith glorified God by being fully assured that he could and would do the impossible.