I read this booklet/essay this past week. Very good. Murray's main point is to steer clear of viewing God's covenant of grace with man in Christ as a compact or contract of mutual agreement. He assumes (doesn't argue for the assumption enough) that viewing the covenant as a contract erodes God's sovereignty in initiating and establishing covenant with man. God has the right to create a covenant with His creation on His own terms, without consulting us or getting us to sign a contract first. He goes through the various covenants in Scripture - with Adam, Noah, Abraham, Moses/Israel, David, showing how God acts unilaterally to establish these covenants. An example, speaking of the covenant at Sinai: "the covenant is not to be regarded as contingent upon the promise of the people, so that the dispensing of the covenant had to wait for this promise.... The covenant had already been established and the blood was simply the confirmation or seal of the covenant established.... th Mosaic covenant also is a sovereign administration of grace, divinely initiated, established, confirmed, and fulfilled" (22).
This does not mean we don't do anything. "The reciprocal response of faith and obedience arises from the nature of the relationship which the covenant contemplates" (18-19). But these are not conditions of the covenant per se, but conditions of remaining in the covenant: "continued enjoyment of this grace and of the relation established is contingent upon the fulfilment of certain conditions.... Keeping the covenant presupposes the covenant relation as established rather than the condition upon which its establishment is contingent" (19).
He is also careful to point out that the central purpose of covenant:
"The spiritual relationship which lay at the centre of the covenant [of] grace disclosed in both the Abrahamic and Mosaic covenants reaches its ripest fruition in the new covenant.... covenant is not only bestowment of grace, not only oath-bound promise, but also relationship with God in that which is the crown and goal of the whole process of religion, namely, union and communion with God" (28, 31).
It's this kind of crazy talk (speaking with tongue in cheek) that has led to the Federal Vision. But that's for another day.