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 4:20–21)
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). “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 4:19–21)
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.