The imputation [of Christ's righteousness] — according to the Reformers, a forensic declaration — was external or nominal in nature
—Hans Boersma, Heavenly Participation: the weaving of a sacramental tapestry (Grand Rapids, MI: Eerdmans, 2011), 92
Among certain segments of Christianity, like the (misguided) "Protestant" appropriation of the Roman Catholic Nouvelle Theologie, the accusation is made that the whole idea of imputation is a nominalist or extrinsic notion. In Boersma's thought, nominalism is one of the bogeyman that caused the crisis of secularism. Frankly, I don't see much of a crisis, and reject the rosy primitivist picture Boersma is painting of a (general) golden age of the "Platonist-Christian synthesis." I don't see much appeal in the "glory days" of superstition, ascetic practices and the promotion of celibacy as a higher form of Christian life (which flow out of any "synthesis"), but if Boersma loves those, I guess he is certainly entitled to his preferences.
The question here however concerns the doctrine of imputation. Boersma makes the claim that the imputation of Christ's righteousness, being forensic, was by nature external or extrinsic, or nominal. Presumably therefore, rejecting nominalism should imply that the imputation of Christ's righteousness should be jettisoned as well. Is imputation therefore considered extrinsic and nominal?
For something to be "extrinsic" means that it pertains to the external circumstances, touching only the surface of a thing, not its nature. For something to be "nominal" means that it is so merely by fiat or naming (hence "nominal"), not by nature. But how does the Scripture portray imputation to be? Imputation touches the nature of a person's relationship to God, and it not a mere change of nomenclature but a true change in one's relationship to God and Christ. If a judge were to acquit someone of a crime, will we say that the acquitted is still "by nature" guilty since the forensic judgment is merely "extrinsic" or "nominal"? Of course not!
The problem with those claiming imputation to be merely external (extrinsic) and/or nominal is that they confuse nature (ontology) with ethics (relations). The two are not the same, nor is one subsumed under the other! A change in one's ethical relation to God is real even though one does not change in nature. Why do people focus so much on "being" (ontology)? The issue with Man has never been principally with being, but with sin (ethics). Yes, punishment for sin in God's curse upon creation does corrupt creaturely being, but as consequence not as cause! Being is never primary in Scripture. The emphasis on Scripture is always ethics (sin), then epistemology (illogicity and ignorance), and then it deals with being only by positing three types of being: Creator, Man, and the rest of Creation.
Imputation is thus not external or nominal in nature, because it is an ethical term, not an ontological term. It has absolutely nothing to do with "being," and unless we start differentiating ethics from metaphysics, we can never truly understand imputation.