William Ames (d. 1633) sounds quite a bit like Francis Turretin (d. 1623) when it comes to the relationship of the covenant of grace and the covenant of works in the old covenant (the Mosaic Covenant).
Ames says the essence (the foundation, the basis, the core) of the covenant of grace is there at Sinai. However, the form (the outward, the visible, the tangible) of it is different from other administrations of it (earlier in the days of Abraham and later in the new covenant). At Sinai, “the form of administration [of the covenant of grace] gave some evidence of the covenant of works.” He later notes that the covenant of grace during the time of Sinai was an “intermixture of the covenant of works, which held the ancient people in a certain bondage.”
Now that we are no longer in the ‘old days’ of the covenant of grace, the form is “essentially different” though the substance/essence is the same. In the covenant of grace today (the new covenant era), the legal bondage is gone because in his saving work, Jesus has done away with it. There is not an ounce of the covenant of works in the covenant of grace today (the new covenant) because of what Jesus has done in gaining our freedom for us.
I think Ames is helpful in this discussion; again, he sounds like Turretin (whom I spoke of before here). If we start blending these things together, what usually happens is that the covenant of works (or some type of merit/works principle) ends up sneaking in the back door of the covenant of grace, and believers are placed under obligation for some aspect of their salvation. Grace is no longer free, and salvation is no longer sola fide. Then, of course, the gospel is at stake and Christian comfort, gratitude, and piety are gone.
The above Ames quotes were taken from his Marrow of Theology, 1.34. As a footnote, am I the only one who thinks the cover of this book (see above) is scary? Couldn’t someone have designed a nicer looking cover?