Here is what Geerhardus Vos said:
It is plain, then, that law-keeping did not figure at that juncture as the meritorious ground of life-inheritance. The latter is based on grace alone, no less emphatically than Paul himself places salvation on that ground. But, while this is so, it might still be objected that law-observance, if not the ground for receiving, is yet made the ground for retention or the privileges inherited. Here it cannot, of course, be denied that a real connection exists. But the Judaizers went wrong in inferring that the connection must be meritorious, that, if Israel keeps the cherished gifts of Jehovah through observance of His law, this must be so, because in strict justice they had earned them. The connection is of a totally different kind. It belongs not to the legal sphere of merit, but to the symbolico-typical sphere of appropriateness of expression (Biblical Theology. Old and New Testaments [Grand Rapids, 1954], p. 143).
John Murray appeals to this quotation in his great essay on law and grace. Every Reformed believer who wants the Reformed baseline on this issue needs to read this essay. As I know from researching my chapter in A Faith That Is Never Alone: A Response to Westminster Seminary California, Murray is here simply applying what he learned from the Reformers and Protestant scholastics.
By the way, when you get to the numbered points 1-4, you might notice a curious resemblance to a series of four posts I wrote.