Critics of the Reformed doctrine of imputation regularly charge that it constitutes a legal fiction. The argument basically argues that to say counting the righteousness of one, namely Christ, in the place of another, namely the guilty sinner, means that the verdict for the sinner does not reflect the sinner's character of life. Thus, God's verdict on the sinner is a matter of fiction; it is not true. He declares someone righteous when that one is actually a sinner. At least three comments are worth making with regard to this charge.
First, critics are right to point out that the verdict does not reflect the sinner's moral quality of life, but this is precisely the point that the Reformed doctrine is making. God justifies the ungodly. If we were already godly and morally righteous, then we would not need to be justified.
Second, confusion comes in that the critics are using the term "righteous" with reference to the moral quality of the sinner. That is, they want the language of righteousness to reflect the person's actual quality of life. The problem is that in the Reformed tradition, the language of righteousness is purely forensic; that is, it does not refer to the individual's quality of life but to his legal status. This happens all the time in courtrooms. Judges and juries find criminals not-guilty. We might respond that injustice has been done, but, at the end of the day, the court's verdict is true as a status. In the eyes of the civic authority, the person is actually not guilty, even if they actually did commit the crime. Again, the imputed righteousness was never intended to refer to the moral quality of the sinner.
Third, critics of the legal fiction variety seem to fail to grasp the covenantal nature of union between Christ and the sinner (perhaps they reject it outright). "Justified" or "righteous" is a status before the divine court. This status is received by virtue of being covenantally joined to Christ. The covenantal nature of the sinner's relationship to Christ means that what is true of Christ is considered true of those who are covenantally related to him. Because "righteous" is a status by virtue of covenantal union, it is not mere legal fiction. The status reflects the verdict of the court as true on the basis of union and communion with Christ. The status was never intended to refer to the moral righteousness of the sinner.
So, the justified status on the basis of the imputed righteousness of Christ is true in that it refers to the declaration of the court. It refers to legal status not moral status, legal righteousness not moral righteousness. If it were intended to refer to the moral quality of the sinner, then it would be legal fiction. Since the verdict was never intended to function in that way, it is not actually fiction. The declaration reflects the reality that the ungodly have been joined to Christ and are sharers in all that is his, including his righteous status before God.