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.
Abraham trusted God even though God gave him no proof, nor even a sign. Rather, there were only mere words promising things which by nature were impossible.
Paul says that Abraham “gave glory to God” when he was attacking those who were seeking their own glory in the sight of men by doing the works of the law.
[AD 407] John Chrysostom on Romans 4:20