Romans 4:19-22 says, "And not weakening in his faith, he considered his own body as already dead, being about a hundred years old, as well as the deadening of Sarah's womb; but with regard to the promise of God, he did not doubt in unbelief, but was empowered by faith, giving glory to God and being fully persuaded that what He had promised He was able also to do. Therefore also it was accounted to him as righteousness."
We should note that Abraham's justification by faith in these verses is different in time from that spoken of in Romans 4:3, which says, "Abraham believed God, and it was accounted to him as righteousness." This is Paul's quotation of Genesis 15:6. It refers to the time before Abraham was eighty-five years old. At that time God spoke to Abraham in a vision: "He that shall come forth out of thine own bowels shall be thine heir." God also brought Abraham forth abroad and asked him to look toward heaven and number the stars and said to him, "So shall thy seed be." Abraham believed in God, and God counted it to him for righteousness. This was the first justification. Although Abraham believed, his faith was not perfect, and later he begot Ishmael by his own flesh. The words "it was accounted to him as righteousness" in Romans 4:22 refer to the incident in Genesis 17. At that time he was ninety-nine years old. Although he considered his own body as being already dead and his wife's womb as being deadened, he did not doubt in unbelief. He fully believed that God would fulfill what He had promised. This was accounted to him as righteousness. Hence, this was a further justification by faith. There was a time lapse of over ten years, but God was still teaching Abraham the same lesson—the lesson of faith. At the beginning, there was Abraham's own element in his faith. After many years he had entirely lost hope in himself, but he was still able to believe. God counted him as righteous based on his faith. God had brought him to the point where he truly believed. This was the result of God's work in him. This shows us that it is not of him who wills, nor of him who runs, but of God who shows mercy (Rom. 9:16). He is the One who initiates the work, and He is the One who carries out the work. May the Lord be merciful to us. May He help us learn the lesson of faith, and may we look to Him alone!