“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. That is why his faith was “counted to him as righteousness.”” -Romans 4:19–22
This short narrative is a recap of Abraham’s faith in preparation to return to Paul’s broader argument, namely that Abraham is the father of those who received righteousness by faith. Paul will return to the present application of faith under the New Covenant.
But here he tells us how Abraham believed. Instead of weakening in faith when he saw his circumstances, his faith in God grew stronger as he gave God glory. He did not waver because looking to God instead of his circumstances he became fully convinced that God was able to do what he promised.
There is something profound here. Paul explains that what happened between God and Abraham is the very reason why his faith was counted to him as righteousness.
This is not the place to debate all the technicalities of the doctrines of synergism vs monergism (God and man working together vs. God working on behalf of both parties)—and we do know that the Genesis narrative to which Paul is referring here shows that God put Abraham to sleep and performed the Covenant sacrifice on behalf of both parties (Genesis 15:4-21)—but Paul is clearly pointing to Abraham’s faith in this passage.
Abraham didn’t believe his circumstances; he believed his God. That is faith. It is a matter of kind and not degree. In other words, Abraham didn’t muster enough faith in order to reach a degree that satisfied God, who in turn, counted it to him for righteousness. Rather, Abraham possessed the kind of faith that sees his truly impossible circumstances and simultaneously sees the glory of God and is even further convinced that God is able. Possessing that kind, and not a certain degree, of faith, God counted it to him as righteousness.