Imputed Righteousness: The Evangelical Doctrine by R.C. Sproul

From Monergism 

  At the heart of the controversy between Roman Catholic and Reformation theology is the nature of justification itself. It is a debate not merely about how or when or by what means a person is justified, but about the very meaning of justification itself.
Reformed theology insists that the biblical doctrine of justification is forensic in nature. What does this mean? In the popular jargon of religion, the word forensic is used infrequently. The word is not foreign, however, to ordinary language. It appears daily in the news media, particularly with reference to criminal investigations and trials. We hear of "forensic evidence" and "forensic medicine" as we listen to the reports of criminologists, coroners, and pathologists. Here the term forensic refers to the judicial system and judicial proceedings.
The term forensic is also used to describe events connected with public speaking. Schools hold forensic contests or events that feature formal debates or the delivery of speeches.
The link between these ordinary usages of forensic and its theological use is that justification has to do with a legal or judicial matter involving some type of declaration. We can reduce its meaning to the concept of legal declaration.
The doctrine of justification involves a legal matter of the highest order. Indeed it is the legal issue on which the sinner stands or falls: his status before the supreme tribunal of God.
When we are summoned to appear before the bar of God's judgment, we face a judgment based on perfect justice. The presiding Judge is himself perfectly just. He is also omniscient, fully aware of our every deed, thought, inclination, and word. Measured by the standard of his canon of righteousness, we face the psalmist's rhetorical question that hints at despair: "If you, LORD, should mark iniquities, ...who could stand?" (Psalm 130:3 NKJV).
The obvious answer to this query is supplied by the Apostle Paul: "There is none righteous, no, not one...." (Romans 3:10).
God commands us to be holy. Our moral obligation coram Deo (before the face of God) is to live perfect lives. One sin mars that obligation and leaves us naked, exposed before divine justice. Once a person sins at all, a perfect record is impossible.

Even if we could live perfectly after that one sin, we would still fail to achieve perfection.

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