Albert Barnes on Romans 4:6

Even as David. The apostle, having adduced the example of Abraham to show that the doctrine which he was defending was not new, and contrary to the Old Testament, proceeds to adduce the case of David also; and to show that he understood the same doctrine of justification without works.

Describeth. Speaks of.

The blessedness. The happiness; or the desirable state or condition.

Unto whom God imputeth righteousness. Whom God treats as righteous, or as entitled to his favour in a way different from his conformity to the law. This is found in Ps 32. And the whole scope and design of the psalm is to show the blessedness of the man who is forgiven, and whose sins are not charged on him, but who is freed from the punishment due to his sins. Being thus pardoned, he is treated as a righteous man. And it is evidently in this sense that the apostle uses the expression "imputeth righteousness," i.e. he does not impute, or charge on the man his sins; he reckons and treats him as a pardoned and righteous man, Ps 32:2. He regards him as one who is forgiven and admitted to his favour, and who is to be treated henceforward as though he had not sinned. That is, he partakes of the benefits of Christ's atonement, so as not henceforward to be treated as a sinner, but as a friend of God.