Obadiah Sedgwick was a member of the Westminster Assembly.  Here is a link to Sedwick's discussion of the Mosaic Covenant.  It is contained in a section which deals with the superiority of the covenant of grace. 

 

His answer to the idea the the Mosaic covenant is a covenant of works can be found here on pg. 172ff.  In the interests of whetting the reader's appetite, we provide a brief summary here. 

 

Like most divines, Sedgwick finds this a confusing issue, and therefore "would speak warily unto it."  Nevertheless, his answer is clear.  With regard to the "intention of God in giving the Law" "God never did, not [sic: should be "nor"] will set up for sinners a Covenant of Works" (173).  These two covenants cannot coexist because they are "mutually destructive one to the other" (173).  Thus, Sedgwick argues that "As God never did (after the fall) make a Covenant of works with sinners, so in particular he did not make such a Covenant with the Israelites when he gave the Law unto them from Mount Sinai, he did not give that Law for to be a Covenant of works..." (174).  

 

Sedgwick then gives seven (very pursuasive, in our opinion) arguments as to why the Sinaitic covenant cannot be a covenant of works.  

 

He then seeks to establish postively the assertion that the Sinai covenant was "a Covenant of Grace for the substance of it" even in a more subservient fashion (175).  This is established by five more arguments. [As a side note, the reader should be careful not to confuse this with the "subservient covenant" position of John Cameron.  Whatever the precise force of Sedgwick's use of the term "subserviently," he is clear that in substance it is a covenant of grace.]  

 

Next Sedwick draws three similarities between the Covenant of Grace under Moses and the New Covenant.  They agree in (1) party, (2) in their mediator, (3) their promise.  This is followed by five differences, all of which concern merely "the partiular form of administration or dispensation," rather than their "substance and end" (177).

 

Sedgwick ends with several practical implications of the doctrine for believers.  While not addressing the theological issue directly, these are important to reflect upon.  This debate (both past and present) is not a mere academic discussion, but has always had significant practical implications for the lives and hearts of God's people.  

 

Enough of my summary--read it for yourself!