And he believed in the LORD; and he counted it to him for righteousness.
Read Chapter 15
Ambrose of Milan
And how did Abraham’s progeny spread? Only through the inheritance he transmitted in virtue of faith. On this basis the faithful are assimilated to heaven, made comparable to the angels, equal to the stars. This is why he said, “So will your descendants be. And Abraham,” the text says, “believed in God.” What exactly did he believe? Prefiguratively he believed that Christ through the incarnation would become his heir. In order that you may know that this was what he believed, the Lord says, “Abraham saw my day and rejoiced.” For this reason “he reckoned it to him as righteousness,” because he did not seek the rational explanation but believed with great promptness of spirit. .
Reputed by God, who cannot judge wrong; so that Abram increased in justice by this act of faith, believing that his wife, now advanced in years, would have a child; from whom others should spring, more numerous than the stars of heaven. (Haydock)
This faith was accompanied and followed by many other acts of virtue, St. James ii. 22. (Worthington)
Accordingly let us learn, I beseech you, a lesson for ourselves as well from the patriarch: Let us believe in the words of God and trust in his promise. Let us not apply the yardstick of our own reasoning but give evidence of deep gratitude. This, you see, will succeed in making us also be seen to be righteous and will quickly cause us to attain to the promise made by him. In Abraham’s case, however, the promise was made that a complete multitude would develop from his descendants. The effect of the promise was beyond the limits of nature and human logic. Hence faith in God won righteousness for him. In our case, … if we are alert enough to see it, he promised much more. We are able in great measure to transcend human reasoning, provided we believe in the power of the One who promises, in order that we may gain also righteousness from faith and attain to the good things promised.