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. — On Abraham Bede : He believed God, and it was reckoned to him as righteousness. The Apostle recalls this sentence (Rom. IV, 3) to commend the grace of God, lest circumcision should boast, and the uncircumcised nations should be unwilling to admit to the faith of Christ: for this was done when faith was reckoned to the believing Abraham for righteousness, and he had not yet been circumcised. Therefore, faith is reckoned as righteousness, not idle and naked, but that which works through love; and also, faith which, although it does not yet have the time to work, still has a perfect will to work. For the faith of the thief was reckoned as righteousness, which, even though it could have no time for works given the imminent article of death, was judged so perfect by the inspector of hearts, that on the same day, it was rewarded with habitation in paradise with him. The faith of Cornelius and his household was reckoned as righteousness, even so much that before the washing of regeneration, they received the gift of the Holy Spirit; because as soon as it was conceived in the heart, it was prepared to work through love. Thus, Abraham's faith was also proven sufficient to be reckoned as righteousness, since he was prepared to offer even his only son as a burnt offering at the command of the Lord. Therefore, the just man lives by faith (Rom. I, 17), that faith surely which is ready to work through love, and if it has time, it works. — Commentary on Genesis (Hexaemeron) John Chrysostom : 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. — HOMILIES ON Genesis 36.15