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.
Ver. 19. “And being not weak in faith, he considered his own body now dead.”
Do you see how he gives the obstacles, as well as the high spirit of the righteous man which surmounts all? “Against hope,” he says, was that which was promised: this is the first obstacle. For Abraham had no other person who had received a son in this way to look to. They that were after him looked to him, but he to no one, save to God only. And this is why he said, “against hope.” Then, “his body now dead.” This is a second. And, “the deadness of Sarah's womb.” This is a third, aye and a fourth obstacle.
Ver. 20. “But he staggered not at the promise of God through unbelief.”
For God neither gave any proof nor made any sign, but there were only bare words promising such things as nature did not hold out any hopes of. Yet still he says, “he staggered not.” He does not say, “He did not disbelieve,” but, “He staggered not,” that is, he neither doubted nor hesitated though the hindrances were so great. From this we learn, that if God promise even countless impossibilities, and he that hears does not receive them, it is not the nature of things that is to blame, but the unreasonableness of him who receives them not. “But was strong in faith.” See the pertinacity of Paul. For since this discourse was about them that work and them that believe, he shows that the believer works more than the other, and requires more power, and great strength, and sustains no common degree of labor. For they counted faith worthless, as having no labor in it. Insisting then upon this, he shows that it is not only he that succeeds in temperance, or any other virtue of this sort, but he that displays faith also who requires even greater power. For as the one needs strength to beat off the reasonings of intemperance, so has the faithful also need of a soul endued with power, that he may thrust aside the suggestions of unbelief. How then did he become “strong?” By trusting the matter, he replies, to faith and not to reasonings: else he had fallen. But how came he to thrive in faith itself? By giving glory to God, he says.
Ver. 21. “And being fully persuaded that what He had promised, He was able also to perform.”
Abstaining then from curious questionings is glorifying God, as indulging in them is transgressing. But if by entering into curious questions, and searching out things below, we fail to glorify Him, much more if we be over curious in the matter of the Lord's generation, shall we suffer to the utmost for our insolence. For if the type of the resurrection is not to be searched into, much less those unutterable and awestriking subjects. And he does not use the word “believed” merely, but, “being fully persuaded.” For such a thing is faith, it is clearer than the demonstration by reasons, and persuades more fully. For it is not possible for another reasoning succeeding to it to shake it afterwards. He indeed that is persuaded with words may have his persuasion altered too by them. But he that stays himself upon faith, has henceforward fortified his hearing against words that may do hurt to it. Having said then, that he was justified by faith, he shows that he glorified God by that faith; which is a thing specially belonging to a good life. For, “Let your light so shine before men, that they may see your good works, and glorify your Father Which is in heaven.” But lo! This is shown also to belong to faith! Again, as works need power, so does faith. For in their case the body often shares the toil, but in the faith the well-doing belongs to the soul alone. And so the labor is greater, since it has no one to share the struggles with it. Do you observe how he shows that all that belonged to works attached to faith in a far greater degree, as having whereof to glory before God—requiring power and labor—and again, glorifying God? And after saying, that “what He had promised, He is able also to perform,” he seems to me to speak beforehand of things to come. For it is not things present merely that He promises, but also things to come. For the present are a type of the other. It is then a sign of a weak, little, and pitiful mind not to believe. And so when any make faith a charge against us, let us make want of faith a charge against them in return, as pitiful, and little-minded, and foolish, and weak, and no better in disposition than asses. For as believing belongs to a lofty and high-born soul, so disbelieving does to a most unreasonable and worthless one, and such as is sunken drowsily (κατενηνεγμένης) into the senselessness of brutes. Therefore having left these, let us imitate the Patriarch, and glorify God as he gave Him glory. And what does it mean, gave Him glory? He held in mind His majesty, His boundless power. And having formed a just conception of Him, he was also “fully persuaded” about His promises.
Source: Homilies on Romans (New Advent)