Basically, the argument is that in the reformed view of imputation, the “righteousness” Christ earns via His keeping of the law is *not a righteousness based on His divine energy (since that is denied in reformed theology). So there are two options for what this “righteousness” He merits in fulfillment of the covenant of works is.
1) Generally, as conceived in reformed theology, it is the works proper to His human nature that are done perfectly in accord with the Law, which legal status is then transferred to us. In reformed theology, as you well know, this is the sole basis for our salvation. Christ is the new Adam who fulfills the covenant of works in which Adam failed. The problem is that all of these works must be the works of his humanity, and therefore are created and temporal – thus the righteousness He earns in this view is a created reality – similar to western Catholic notions of “created grace.” The works Christ does, then, are not strictly and solely the works of a single divine Person. This is seen clearly in the normative reformed doctrine of the Father’s damning of the Son, which is anti-Trinitarian to the core.
For this to occur, since most reformed people don’t want to split the Trinity up, they must confess a human subject or person, Jesus of Nazareth, who is the subject of this damnation by the Father. So then the legal exchange is that a human subject keeps the law and fulfills what Adam failed in (the covenant of works), giving rise to a *created legal state (since the legal state is *not based on Christ’s divine status as Son and His divine energy) which is transferred to us, while our damnation/spiritual death is transferred to a human Person Jesus (which is Arian/Nestorian). Thus, what saves us is ultimately a creature – a created legal state. But Jesus isn’t a human person and what we get in salvation is not another created state, but eternal life – the very glory He shared with the Father from the beginning. That glory is obviously not a creature. Therefore salvation cannot be a created legal state. As Steven Kaster writes, explaining the western (Thomistic and Protestant) notions of created grace and divine simplicity:
“. . . God is defined only in terms of His essence [in Western Dogmatics]; whatever is not essence does not belong to God; it is a creature of God, the result of the divine essence. Consequently, the energies [operations] of God are either identified with essence, which is active (actus purus), or else any external manifestation of theirs is regarded as necessarily hetero-essential, i.e., a created result of the divine cause,” and as Yannaras goes on to point out, “This means that, in the final analysis, the theosis of man, his participation in the divine life, is impossible, since even grace, the sanctifier of the saints, is itself an effect, a result of the divine essence. It is created, even though supernatural, as western theologians have rather arbitrarily defined it since the ninth century.” To hold that created realities can bridge the gap between the uncreated God and His creation is to fall back into a form of Arianism, which held that the Son Himself was created. The only difference between Arianism and the more modern idea of created grace, is that the Son is affirmed as co-essential with the Father and thus He is uncreated, but the grace that He bestows is somehow held to be a created reality; and so once again, like the Arians, it is a created being or substance that acts as the intermediary between God and man, and man’s deification becomes impossible because a created reality simply cannot deify man.”
Kaster is here concerned with the relation of created grace to the Trinity’s operations in the economy, but I am applying this same idea to the reformed view of imputed righteousness and soteriology. The righteousness Christ merits in this view has to be temporal and created, since, in this view, the human-person-Jesus does these good “works” in time and according to His humanity, and because of the reformed acceptance of absolute divine simplicity. To my knowledge, because no reformed person makes a distinction between God’s essence and His actions (just like in Thomism) a reformed person must argue that this is a created, temporal “righteousness” transferred to us as the basis of our justification. But a creature is not what saves us (that’s Arian) – we need divine life: immortality (1 Cor. 15). And divine life is not created.
2) The other possible option (which no reformed person I’ve ever heard of holds to) would be to say that Christ did His human works according to the divine nature. In other words, the righteousness He merits is a perfect righteousness because it has the quality of the divine nature, somehow. But Jesus’ operations in His humanity are not the divine ousia and cannot be. The divine nature is not like anything created (Acts 17:29) and cannot be participated in, considered in itself. It cannot be looked upon or experienced or comprehended (Ex. 33). And, again, no reformed person would argue this way.
So the argument here is just a variation on the numerous problems associated with western notions of created grace – that what we get in salvation is merely a created “substance” or state. Whereas Catholic theology has tended to say what we “get” in salvation is a created “substance” as a causal effect from the absolutely simple essence, Calvinistic imputational theology must say that what we “get” is merely a created “legal state” based on the created legal state earned by the human-person-Jesus-of-Nazareth. But this denies that the Word – the second Person of the Godhead – is the sole subject of all the actions of the Incarnate One. Remember – classical Christology says Jesus is a divine Person with two nature and two operations (energies).