To embrace covenantal nomism as an appropriate system for the national covenant of Israel is, I have argued, properly to intepret the old covenant history. However, to embrace it as an appropriate system for answering the question as to how an individual is rightly related to God remains, as it has always been, a confusion of Abraham with Moses, which Paul relates as a confusion of Sarah with Hagar, freedom with bondage, Sinai with Zion, and law with gospel.
To put it differently, it is appropriate to treat the earthly promises (land, temple, kingdom) as conditioned upon the covenant people's personal obedience to the law, but fatal in Paul's view to treat the heavenly promises (the new creation, Christ, and his everlasting Davidic reign) as conditioned on the obedience of anyone other than Christ himself.
It is certainly wrong to characterize Judaism as mere legalism, much less the Old Testament as a religion of works-righteousness, but it is just as wrong from a Christian perspective to characterize synergism (covenantal nomism) as a legitimate description of the covenant of grace when it is as obvious from the Hebrew narratives as it is from their New Testament interpretation that the Abrahamic inheritance comes by grace apart from works, by promise apart from law.
Michael Horton, Covenant and Salvation: Union with Christ, p. 50-1