What does it mean that Abraham was fully persuaded in Romans 4:21?

The promise made to Abraham was that he would be the father of many nations. It had been given a long time before, and a long stretch of that time had gone by with nothing to show. He was about a hundred years old and Sarah was ninety; she had never borne a child at any age, and there was still no child of hers. As far as fathering a child went, his body was as good as dead, and his wife's with it.

Paul spends the whole passage describing Abraham's faith and commending it, and he starts with what that faith had its eye on: "God, who quickeneth the dead" — who gives life to the dead. A man promised children when he and his wife are past having them is looking at the God who can put life into what has none left. The difficulty here was two worn-out bodies. So faith fastened on the thing in God that answered exactly that difficulty: he raises the dead. Faith looks at the strength of God, not at the way things normally work and not at the obstacles that look impossible to a man. He is almighty and unchanging, and he raises the dead. And what he promises is so certain that he can speak of it as already standing: he calls things that are not as though they were. He is the Creator: he gave being where there was none, so he can guarantee the existence of a thing that is not yet, and he can never fail to do what he has said.

Here is how Paul puts it: "Being not weak in faith, he considered not his own body." He did not dwell on it in his thoughts. That is not the same as pretending it was not there. He may well have turned the thing over in his mind; what he did not do was let it settle there, or look at it long enough to start distrusting the promise. He did ask the question about himself — Shall a child be born to him that is a hundred years old? — but that was the language of amazement, and of a man wanting to know more, not of doubt and distrust. His faith went straight past the state of his body and thought about nothing except the faithfulness of the promise.

"He staggered not at the promise of God through unbelief," Paul writes. Staggered there means hesitated. He held no private debate about it and did not take time to weigh up whether to accept it or not; he settled it as a matter already ruled on, and staked everything he had on it. God's own people do stagger at his promises — some of them reckoning a promise too great for the likes of them, others holding back because of the difficulties attached to it. Abraham did not. The reason there is nothing to hesitate over lies in the promises themselves: they are made by one who cannot lie, his faithfulness is engaged to keep them, and nothing is impossible with him.

Abraham could see the storms of doubt and fear and temptation likely to get up against the promise. Plenty of men would have shrunk back at that and pulled in to wait for better weather — for the day when the thing finally looked reasonable. He took God for his pilot and the promise for his chart and compass, put up every sail he had, and made for the harbour.

What Abraham was persuaded of was not anything about himself. It was that God was able, being the Lord God Almighty, to perform what he had said, after all that time and with so many difficulties in the way. That is exactly where his faith gave God glory — he was firm in the persuasion that God was able to fulfil his promise, and so no difficulties shook him. Our wavering rises mainly out of doubting what God can do. So what steadies a person is not only that the God who made the promise is faithful, but that he is able. And Abraham was certain that God not only could, but would, do what he had promised.

Look at the shape of the delay. Abraham could not be the father of nations without a son, and what died was every means of getting one. The son did come, in the end — Isaac, raised up out of a womb as dead as Abraham's own body.

Paul draws his conclusion from the believing, not from the child. "And therefore it was imputed to him for righteousness" — imputed meaning reckoned, put to his account, so that he was regarded and treated as righteous. He had staked everything he had on a bare promise. Resting everything on a bare promise was the way God had chosen to justify people. So God graciously accepted that reliance as a righteousness, and justified him. It was not counted to him because of anything he deserved.

The believing did not pay for anything. There was nothing in the reliance itself that could have merited an acceptance like that. Faith is merely the instrument that puts a person in possession of a blessing given free — the means God appointed, by which a man may be treated as righteous. What Abraham's faith laid hold of was the promise, and inside the promise the Saviour who was still to come; the merit was the Saviour's, and not Abraham's. And because faith is the same thing in nature and in kind in every true believer, differing only in degree, a weak faith is counted the same righteousness as a strong one.

He did not get younger while he waited, and his wife did not, and the promise was never cut down to fit what was left of them. Among men we say that a man who trusts another honours him by taking his word. That is what an old man with nothing on his side of it did — and what was counted to him for righteousness was not how sure he had managed to be, but who it was he was sure of.