As always, when the apostle Paul talks about faith, he adds hope as well, and rightly so, for hope and faith are inseparable.… Just as Abraham believed against hope, so all believers do the same, for we all believe in the resurrection of the dead and the inheritance of the kingdom of heaven. These appear to go against hope as far as human nature is concerned, but when we take the power of God into consideration, there is no problem.
It is clear that since Abraham had no hope of having a son, he believed God and had faith against hope that he would have a son, knowing that with God all things are possible.The quotation is from Genesis [15:5], where God shows Abraham the stars of the sky and says: “So shall your descendants be,” because in believing he was justified. For Abraham believed what seems impossible to the world because it does not occur in the order of nature that old people should have children and know that their seed will increase to such an extent that it will be impossible to count them. Therefore, faith is precious because it believes in the future, even against what it now sees or knows. For it consoles itself in this hope, that it is God who promises.
How was it that he "believed in hope against hope?" It was against man's hope, in hope which is of God. (For he is showing the loftiness of the action, and leaving no room for disbelieving what is said.) Things which are contrary to one another, yet faith blends them together. But if he were speaking about such as were from Ishmael, this language would be superfluous: for it was not by faith but by nature that they were begotten. But he brings Isaac also before us. For it was not concerning those nations that he believed, but concerning him who was to be from his barren wife. If then it be a reward to be father of many nations, it would be so of those nations clearly of whom he so believed. For that you may know that he is speaking of them, listen to what follows. (Romans 4:19)
It was against all natural hope for a hundred-year-old man to believe that his seed would become as numerous as the stars, especially given that his wife had been barren in her youth and was now as feeble as he was.
[AD 253] Origen of Alexandria on Romans 4:18