What exactly is so threatening about this?
Every Reformed minister loves preaching from Romans and Galatians. Presenting the Mosaic law as teaching a works principle really helps in explaining Paul’s doctrine of justification: what sin is all about, why people can’t rely on their own law-keeping, how faith is radically different from works, how Christ fulfilled the terms of the law so that we may be justified. That’s the gospel as I see it, but you can’t explain the gospel without understanding the law. Or take all of those Old Testament passages that call for Israel’s obedience and promise blessing and threaten curse in the land depending on their response. For example, the beginning of Deuteronomy 4, which tells Israel to follow the law so that they may live and take possession of the land. Or Deuteronomy 28, which recounts all sorts of earthly blessings in the land if the Israelites are careful to obey and all sorts of earthly curses if they aren’t. I don’t want a congregation to think that God was holding out a works-based way of salvation here, and I also can’t tell the congregation that this is the same way that God deals with the New Testament church when he calls her to obedience, for there’s nothing equivalent in the New Testament, no promise of earthly blessing for the church today if we meet a standard of obedience. Saying either of those things might by simple, but of course they’d be misleading, and damaging for the church to hear. (The Law is Not of Faith, 5)
Could it be that this view seems to allow Christians to think that law-keeping does not contribute to their salvation? Well, if the law requires “personal, perfect, and perpetual conformity and obedience thereunto, in the frame and disposition of the whole man, soul and body, and in performance of all those duties of holiness and righteousness which he owes to God and man: promising life upon the fulfilling, and threatening death upon the breach of it,” who is up to that challenge? Don’t be bashful.