And it was therefore imputed, [1] etc. It becomes now more clear, how and in what manner faith brought righteousness to Abraham; and that was, because he, leaning on God's word, rejected not the promised favor. And this connection of faith with the word ought to be well understood and carefully remembered; for faith can bring us nothing more than what it receives from the word. Hence he does not become immediately just, who is imbued only with a general and confused idea that God is true, except he reposes on the promise of his favor.

Footnotes

1 - As in a former instance in Romans 4:3, there is no nominative case to this verb: it is supplied by the sentence. This is the case not unfrequently in languages, such as Greek and Hebrew, in which the person is included in the verb itself. There is no nominative in the Welsh version, and there seems to be no need of it, Amhyny y cyvrivwyd iddo yn gyviawnder "It is most true, as Paul says to the Romans, that by faith Abraham was justified, and not by obedience: but it is just as true what he says to the Hebrews, that it was by faith that Abraham obeyed." -- Chalmers.

And therefore - His faith was so implicit, and so unwavering, that it was a demonstration that he was the firm friend of God. He was tried, and he had such confidence in God that he showed that he was supremely attached to him, and would obey and serve him. This was reckoned as a full proof of friendship; and he was recognised and treated as righteous; that is, as the friend of God. (The true sense of faith being imputed for righteousness is given in a note at the beginning of the chapter.) See the note at Romans 4:3, 5.

And therefore it was imputed to him for righteousness - The verse is thus paraphrased by Dr. Taylor: "For which reason God was graciously pleased to place his faith to his account; and to allow his fiducial reliance upon the Divine goodness, power, and faithfulness, for a title to the Divine blessing, which, otherwise, having been an idolater, he had no right to."
Abraham's strong faith in the promise of the coming Savior, for this was essential to his faith, was reckoned to him for justification: for it is not said that any righteousness, either his own, or that of another, was imputed or reckoned to him for justification; but it, i.e. his faith in God. His faith was fully persuaded of the most merciful intentions of God's goodness; and this, which, in effect, laid hold on Jesus Christ, the future Savior, was the means of his justification; being reckoned unto him in the place of personal righteousness, because it laid hold on the merit of Him who died to make an atonement for our offenses, and rose again for our justification.

And therefore it was imputed to him for righteousness. Not because his faith was strong, and he had a full assurance of it, but because it was right, resting on the promise of God, and relying upon his power and faithfulness to perform it; for though the righteousness of faith is not imputed to any sort of believers, not to mere nominal ones, yet to all such as have true faith, though it may be but weak; for faith, as to nature, kind, and object, though not as to degree, is the same in all true believers, and the same righteousness is imputed to one as to another.

And therefore it was imputed, &c.--"Let all then take notice that this was not because of anything meritorious in Abraham, but merely because he so believed."