So also David speaks of the blessedness of those to whom God reckons righteousness apart from works. "Blessed are those whose iniquities are forgiven, and whose sins are covered; blessed is the one against whom the Lord will not reckon sin." (Rom 4:6-8, NRSV)
Rom 4:6-8 is one of the best passages refuting Reformed theology. For a discussion, see:
King David Refutes Reformed Soteriology (cf. An Examination and Critique of the Theological Presuppositions Underlying Reformed Theology)
Commenting on the exegetical method Paul is using, Susann M. Liubinskas:
[I]n verses 6-8, he applies the Rabbinic exegetical principle of gezerah sawah, interpreting Gen 15:6 in light of Ps 32:1-2; in effect, stating that the reckoning of righteousness is equivalent to the nonreckoning of sins. Of note is Paul’s explicit reference to David, another key founding figure from Jewish history, who also has eschatological significance, since it is from his line that the Messiah was expected to come. In the psalm that Paul makes reference to, emphasis is placed on the psalmist’s acknowledgment and confession of transgression and God’s freely given forgiveness. There is no mention of the sacrificial system of atonement, as prescribed by the law. Rather, Paul refers to David as one who experienced blessedness as God’s reckoning of righteousness apart from works (4:6). This anticipates and recalls the eschatological promise that God would freely forgive Israel’s sin, if its people should repent, suggesting that, although the law provided a system for atoning for sin, the forgiveness of this sin ultimately resides in God who is free to establish any system of atonement he sees fit, including the present sacrificial atonement of Christ (3:24-26). Recall that Paul has already referred to God’s forbearance in passing over previously committed sins until the present (3:25-26), pointing to the penultimate status of the levitical sacrificial system of atonement. As Abraham was reckoned righteous due to his trust that God forgives the repentant sinner. Moreover, in the case of the latter, whose double sin of premeditated adultery and murder could not be atoned for under the Mosaic law (Acts 13:39), simultaneously underscores the degree of reliance on or faith in God that this founding figure exemplifies, particularly in light of his dire situation, and the mercy and faithfulness of the God upon whom he relies and in whom he trusts (both the theme of trust in God in dire circumstances and that of the infinite mercy and faithfulness of this righteous God are brought up by Paul time and time again as his arguments proceed. Moreover, both of these themes relate to his larger ethnographic aims). In this way, Paul draws a line from Abraham to David and, ultimately, to Christ via faith or trust in God as the one who both establishes and restores his people. Faith or truth in this God, then, is the trans-historical criterion identifying the people of God. (Susann M. Liubinskas, The Ethnographic Character of Romans: The Dichotomies of Law-Faith and Jew-Gentile in Light of Greco-Roman and Hellenistic Jewish Ethnography [Eugene, Oreg.: Pickwick Publications, 2019], 141)