A very common objection from Roman Catholics against the Protestant doctrine of imputation is that God declares someone to be innocent who is not, in fact, innocent. This is legal nonsense, to them. They believe that God would never declare a person to be righteous who is not, in fact, righteous. So, the Protestant idea that an alien righteousness, that of Christ, is reckoned to the sinner, is nonsense to them. It would be God declaring something to be true which is actually false. So, how do Protestants respond to this? There are a variety of responses, but the best one, it seems to me, resides in the metaphor of marriage union. We will also add a few things afterwards that will help us understand.
In most marriages, property entails joint ownership. Now, if a woman comes into the marriage with a debt (like a college debt), the husband assumes that debt. It becomes their debt (it can also be described as his debt), even though the husband did not incur that debt. Similarly, whatever money the husband brought into the marriage doesn’t belong just to him anymore, it also belongs to her, even though she did not earn it. So, by virtue of the marriage union between husband and wife, the debts and the assets are transferred.
In a very similar way, when the believer becomes united to Christ by faith, a new legal situation results with transfers happening. I think a lot of the problems that Roman Catholics have over the Protestant doctrine is that sometimes Protestants formulate the alien righteousness imputation idea as though there were no other accompanying-but-distinct salvific benefits happening at all.
Now, let us be clear here. The Protestant doctrine should never be formulated in such a way that union with Christ, for instance, has an internal change happening in the believer that thereby becomes the basis for the imputation. Christ’s righteousness is the basis for the transfer, not anything that happens in the believer. It happens by the instrumentation of faith.
What the marriage union does accomplish in justification has to do with the legality of the transfer. The new legal status we have as being part of the bride of Christ (our being married to Christ) means that anything Christ transfers to us happens legally whether we deserve it or not.
There is, of course, another concomitant salvific benefit that has equal power to explain how it is that justification is not a legal fiction. This benefit works on a different level, but it is still quite effective in combating the “legal fiction” charge, and it is just as biblical. When the believer comes to faith in Christ, he is adopted as God’s child. Adoption also confers a new legal status, this time with more reference to the Father through Jesus (whereas the marriage happens between Christ and the church, adoption is more the Father’s action, though it certainly has reference to the Son and the Spirit of adoption). When God declares us His heirs, then there is no reason whatsoever that God can not transfer anything to us that originally belonged to His Son. It would be no more difficult than imagining a father changing his will.
One last distinction can help us here. There is a difference between being a sinner as opposed to what our legal status before the law is. With regard to being a sinner, we are always sinners until God takes away our sin nature in death. However, with regard to our legal status before the law, God’s declarative action makes us legally innocent, even though that declaration does not change our nature. So when Luther says simul justus et peccator (simultaneously just and a sinner), we are to understand that we are still sinners (though we have been changed in regeneration such that being a sinner is not all there is to say) in our being, and yet we are actually just in the view of the law. Our legal status has similarities to a criminal who is acquitted of a crime that he did in fact commit. He is in his being guilty, and yet in the eyes of the law, he is not guilty.
While Roman Catholics will certainly not agree with these formulations, nevertheless, I believe that the above does put to rest the rather old canard that Protestants believe in a legal fiction in the doctrine of imputation. Marriage and adoption create new legal situations where transfers are not only easily accomplished, but are in fact rather normal.