In his Systematic Theology, Robert Letham expresssed humanity in covenant as purely one of grace. Accordingly, Letham denies the Covenant of Works, expressing the fact that God deals with Man only in a gracious manner.
It is of course true that law and grace can be seen as congruous with each other, but only in two senses. One, the Old Testament (law) is congruous with the New Testament (Grace). Secondly, as I have written in my response to David Cheng on the issue (Daniel H Chew, "Response to 'Calvin on Law and Grace," in Daniel H Chew and Jonah Tang, eds, Faith Seeking Understanding - Vol. 2: The Reformation and John Calvin - Proceedings from the 2009 CREDO500 Conference), the law as what one practices is congruous with God's grace. However, there is a third sense in which law and grace (or rather Gospel) is not congruous but diametrically opposed to each other, and one that lies behind the bicovenantal structure of the Reformed tradition. I will call the first sense, the historical sense, Law1 and Grace1. The second sense of practice I will call Law2 and Grace2, and the third sense, the antithetical sense, Law3 and Gospel. Letham embraces the first two senses, but either does not understand or deny the third sense.
On page 356, Letham asserts, citing WCF 7.5, that "grace and law are not competing ways of salvation; they fulfill different functions: grace constitutes, law regulates." In response, it must be stated that in WCF 7.5, "law" and "gospel" was used in the historic sense as signifying the Old and New Testament economies. But what Letham has done is to conflate Law1 with Law3, and conflate Law2 with Law3. Since WCF 7.5 deals with the historical senses, they cannot be used for the other two senses of the terms. Letham's statement basically flirts with Federal Vision, since law even in regulating the Christian life does NOT function as a way of salvation. Compounding his error is the statement that "by extension, grace and law are also present in the covenant of life" (p. 356), which is a non sequitur.
Under the section of "theological considerations," Letham has other arguments for his monocovenantal position. In page 359, Letham argues, "If God related to Adam in creation exclusively by law, our view of the covenant of grace willl be dominantly legal, for grace will serve the purposes of law, and this will color the way we relate to God now." This however is only true in a monocovenantal framework, where what is true of the Covenant of Works is also true of the Covenant of Grace. In page 362, Letham states that "if God had related to man in creation exclusively by law, then his revelation of himself to Adam would not have been a true self-revelation at all, and creation would not express the character of God." This is errant because God did not just give the Covenant of Works to Adam but also the entire garden of Eden, his wife Eve, and communion with God Himself.
Perhaps the strangest argument is seen in page 361, where Letham wrote:
Advocates of the view that the Adamic covenant was purely legal argue that if grace was present, merit would necessarily have been denied, consequently threatening the second Adam's meritorious obedience. This concern stems from a polarized view of law and grace, and puts law prior to grace. It creates precsiely the problem John McLeod Campbell (1800-1872) faced. Campbell argued that Scottish Calvinism had made justice an essential attribute of God and love purely arbitrary. The consequence, he argued, was the priority of the legal over the filial, with devastating effects on piety and assurance. (p. 361)
In response, it must be stated that the argument is one big non sequitur. First, what does the issue of a legal Adamic covenant have to do with the attributes of God and whether justice and/or love is an essential attribute? Second, since Campbell was deemed a heretic by the Scottish church of his time, why should anyone care about Campbell's view on Scottish Calvinism? Third, how does a legal Adamic Covenant result in putting "law prior to grace"? What does this "law prior to grace" even mean?
The proponents for the Covenant of Works argue for the parallelism between Adam and Christ based upon biblical texts such as Rom. 5:12-21. When we argue for why the Covenant of Works must be a covenant "of works," we do see a denial of the works principle here as threatining the second Adam's meritorious obedience precisely because of these texts. Therefore, Letham's hand-waving as saying that such an argument "clouds the issue and makes meaningful discourse difficult" (p. 361) is special pleading.
In conclusion, Letham's arguments for a gracious Adamic Covenant fail. He conflates the various senses of law and grace as used in the Reformed confessions. He also fails to recognize and understand the monocovenantal understanding he is operating upon, and thus there is no critical engagement on this issue. Lastly, he fails to deal with certain key biblical texts concerning the Adamic Covenant. Letham's view on the Covenant of Life is therefore not only in error but also unsupported and poorly argued.