Yet there are challenges to this distinction, which moves the chapter in the direction of a didactic polemic. In answering the first challenge that justification is merely the forgiveness of sins without the need for the imputation of Christ's righteousness, Fesko writes:
...we may say that God..initially judged Adam after his creation and declared him righteous; nevertheless he also set before him the work of the covenant. Upon completion of that work, God would have once again declared him righteous. He would have been righteous by both, negatively, abstaining from the fruit of the tree of knowledge, and positively, fulfilling the requirements of the dominion mandate. It is evident then, that man requires both the absence of sin and the presence of righteousness, or obedience to the law, to be declared just by God. (p152)
Those who would argue that one cannot make a distinction between active and passive obedience because Scripture views Christ's obedience as all of a piece, are not really paying attention to the purely pragmatic nature of the distinction. No one who holds the distinction would deny that all of Christ's obedience was active, e.g. on the cross Christ dismissed his own spirit. Yet it is merely a useful distinction in order to distinguish Christ's positive fulfillment of the law from his atoning death.
Holding to the covenant of works as fulfilled by the last Adam where the first Adam failed is necessary to safeguard justification apart from works:
If one does not account for the failed obedience of the first Adam, which completely closes off the path to justification by works for fallen man, then he will likely reintroduce works into postfall justification. Justification will not be by faith alone in Christ alone, grounded upon the obedience of Christ, but instead will be a combination of faith and obedience, or works. (p157)
The covenant of works is not just some quaint and dusty piece of Reformed theology that we can admire with a patronizing air of biblical/theological superiority. It is at the very heart of the Reformed doctrine of justification.