I have loved this quotation from Vos as soon as I read it:
Before all other important things, therefore, the Sabbath is an expression of the eschatological principle on which the life of humanity has been constructed…The Sabbath brings this principle of the eschatological structure of history to bear upon the mind of man after a symbolical and a typical fashion. It teaches its lesson through the rhythmical succession of six days of labour and one ensuing day of rest in each successive week. Man is reminded in this way that life is not an aimless existence, that a goal lies beyond. This was true before, and apart from, redemption. The eschatological is an older strand in revelation than the soteric. The so-called ‘Covenant of Works’was nothing but an embodiment of the Sabbatical principle. (from Biblical Theology, p. 140).
It just struck me recently that the Covenant of Works functions as a Sabbatical principle because of the work-rest paradigm of the Covenant of Works. As God had rested from His labors, so also was Adam going to rest from his labors, had he obeyed.
This eschatological Sabbath-structure of the Covenant of Works plays also into justification and the Covenant of Grace, in that Christ has done the work while we get the rest of eternal life. Of course, this is only true in an “already” sense. There still remains a Sabbath rest for the people of God, which is why the Sabbath has not been abrogated. But the Sabbath character of the Covenant of Works is why all attempts to make Adam’s obtaining of eternal life solely by grace through faith fall to the ground, whereas Christ’s obtaining of that Sabbath rest for us (in an already/not yet schema) is the fulfillment of the Sabbatical principle of the Covenant of Works in the Covenant of Grace.