[Foggy Harbor, 2015]
Romans 8:3–4 (NKJV) 3 For what the law could not do in that it was weak through the flesh, God did by sending His own Son in the likeness of sinful flesh, on account of sin: He condemned sin in the flesh, 4 that the righteous requirement of the law might be fulfilled in us who do not walk according to the flesh but according to the Spirit.
Herman Witsius (1636-1708) wrote the best summary development of the relation between the Covenant of Works and the Covenant of Grace, and specifically how the abrogation of the first did not absolve man, even redeemed man, from the requirement of obedience to God's law, that I have heretofore encountered. (The Economy of the Covenants, BK. I, ch. 10 in particular)
This topic is a matter of some passion these days. The discussion ranges over topics such as how we understand the "Mosaic Economy" (a republication of the covenant of works for national Israel?), "sanctification" (is it a work of God on the mostly passive redeemed sinner?), "Christian ethics" (is there any such thing?) and a host of others.
What I find interesting is that Witsius' 17th century Christian contemporaries were pretty much occupied with the same debate. Indeed, Witsius wrote his work in hopes that he might furnish grounds for bringing the sides together. He was perhaps naive in this then and I don't see any reason for great optimism now.
What I do see is that Witsius' arguments, as a sustained incremental piecing together of the various Biblical texts and their most reasonable interpretations, is conclusive. I offer this short quote as summary.
It is indeed a most destructive heresy to maintain that man, sinful and obnoxious to punishment, is not bound to obedience. For by no misconduct of man can God forfeit his right and supremacy. But the right and supremacy of God requires that man, and even every creature, be subject in all respects to God, so far as possible. Moreover, the rational creature, such as sinful man is, and does continue to be, can be subject, not only to the natural, but also to the moral providence of God; nor only to his vindictive justice, but also to his legislative authority: and as he can, so he ought to be subject to him, as to the obligation of obedience; because every possible subjection is essential to the creature.
Witsius' sustained argument is too long to reproduce but he grounds his conclusions on the nature of God Himself and the Divine will that all things lead to His own glory. Man, before the covenant of works was established and the potential for eternal reward set forth, was yet created under the obligation to obey. Obedience, in and of itself, is inherent in the Creator-creature relation. Witsius moves on to discuss the differences in the kind of obedience God requires of His human subjects and notices that there is that obedience to God's will that reflects the character of God Himself and that obedience (probationary) God requires in order to "test" or "prove" or "discipline" those subjects. This last category is not arbitrary in the sense that God requires it in a whimsical fashion, it is always designed perfectly to accomplish its own ends. The point is that God may, with no insult to His own Person, change this latter type of required obedience or discard it completely, while such is not the case with the first. Thus the "law of God" is comprised of categories, some of which are inalienable to Himself and His glory and others which are not. The natural obedience owed by the creature is directed to God in both categories and the Covenant of Works then follows as God acting with our first parents to institute a formal relation built on this obedience but, critically, adding to it a promise: "If you do this, you shall live." God's Son, Jesus of Nazareth, born under the law of creation, undertook the Covenant of Redemption in its eternal form: "If you do this, then You and all the elect in You shall live", though the "this" in the two covenants differed in their probationary aspects. In the Covenant of Grace, which God has extended to the redeemed in every dispensation, the formulation was no longer "if you do this, you shall live", but rather "the law no longer condemns you, but you still owe obedience from the heart to your Creator, Redeemer and Friend."
Witsius' treatment is exhaustive (and perhaps exhausting to those less motivated) on the implications of this development but it is well worth reading. He pretty much undermines the notion that the Mosaic economy was a republication of the Covenant of Works by his exegesis of Heb. 8:13. The Covenant of Grace did not "abrogate" the Covenant of Works but rather "presupposes" that that covenant was already abrogated. Having abrogated it already, God, in Whom there is no shadow of turning, would not re institute it even in a partial form. Every covenant has a "promise", indeed the purpose of God's covenants are not to introduce the idea of obedience, that is already present in the Creator/creature relation. Rather, God's covenants are all "gracious" in the restricted sense that, though under no obligation to do so, God gives man a means by which further and greater blessings may accrue. Man, having failed in his first probation, having lost all ability to will or do perfectly anything that God requires, cannot, either corporately or individually, ever have hope of any promise of meritorious blessing which is tied to his personal obedience. Thus, for the Mosaic covenant to be a republication of the Covenant of Works, corporate Israel would have to have at least the potential of fulfilling the law. This potential was entirely absent.
There is that sense of "do this and you shall live" which is entirely consistent with "believe and you shall be saved." The difference between them is the difference between promises of the Covenant of Works and the promises of the Covenant of Grace. The Mosaic covenant, as does the Gospel, condemns those who undertake to obey apart from faith. However, when works proceed from faith, from believing, then "do this and you shall live" becomes the reality of eternal life which is entered into even now.
Thus, when viewed, rightly, as being a progressive development of the Covenant of Grace, all difficulty is removed. The Siniatic covenant presupposes Divinely mediated grace. It presupposes a "substitute" for man's sin such that "looking" to that Substitute and Mediator through the Levitical sacraments, constituted the hope of salvation and the predestined fulfillment of the Covenant of Redemption when the Divinely provided Ram (Abraham with Isaac) would appear and fulfill all righteousness.
Witsius saw clearly ... and his work abides as a refreshing and re-energizing message to Christ's Church and a call to arms to defend the Orthodox Reformed Confession, and to the call to obedience, in faith, as a signature distinctive of a regenerate heart.
 (Witsius, Economy, Bk. I, 2010) Logos, p. 125