4:7 Saying. This quote is from Psalm 32:1-2. This was David’s psalm of thanksgiving after his repentance over his notorious sin of adultery and homicide. Paul thus notes that justification by faith was true both before and after Moses—before, in Abraham, Israel’s great patriarch, and after, in David, Israel’s greatest king.
4:8 not impute sin. By the marvelous provision of imputation, our sins were debited to the account of Jesus, the Son of man, whereas His perfect righteousness was credited to our account. “For He hath made him to be sin for us, who knew no sin, that we might be made the righteousness of God in Him” (II Corinthians 5:21). See also James 2:23; Philemon 17-18.