The third aspect of Abraham’s Faith which Paul examines in Romans 4 is that Abraham “was fully convinced that God is able to do whatever He promises.” Romans 4:21 NLT
“Even when there was no reason for hope, Abraham kept hoping– believing that he would become the father of many nations. For God had said to him ‘That’s how many descendants you will have!'”“And Abraham’s faith did not weaken, even though, at 100 years of age, he figured his body was as good as dead– and so was Sarah’s womb. Abraham never wavered in believing God’s promise. In fact, his faith grew stronger, and in this, he brought glory to God.” Romans 4:18-20 NLT
God saw this faith of Abraham, his threefold realization of God’s own character, and the robustness with which he believed in God’s promise, in spite of all appearances to the contrary. And, seeing his faith, God counted him as righteous.
As Charles Spurgeon says in his book All of Grace, faith is believing that Christ is who He is said to be, that He will do what He has promised, and then expecting this of Him. We see this clearly spelled out in Paul’s description of Abraham; He saw God, raising the dead, creating out of nothing by His word, and fully able to do whatever He had promised. So Abraham, looking around at the impossibilities, nevertheless believed God’s Promise. And as he believed, his faith grew stronger and stronger, he brought glory to God, and God, seeing his faith, declared him righteous.