Robertson argues that "the covenants of Abraham, Moses, and David actually are successive stages of a single covenant" (41). I'm not going to unpack out his whole case, but I trust it's reasonably obvious how this conclusion also serves as a necessary premise for fundamental continuity in God's relationships with mankind after the fall. IOW, if those three covenants aren't essentially the same covenant, the CT argument for strong continuity can't stand.
Here are a couple other examples of this piece of the CT argument:
Jeremiah's classic prophecy clearly relates the new covenant to its Mosaic predecessor (cf. Jer. 31:3ff.). This "new covenant" with the "house of Israel and with the house of Judah" will not be like the Mosaic covenant in its externalistic features. But the law of God as revealed to Moses shall be written on the heart. While the substance of the law will be the same, the mode of its administration will be different. The form may change, but the essence of the new covenant of Jeremiah's prophecy relates directly to the law-covenant made at Sinai. (41). . . and . . .
The covenants of God are one. The recurring summation of the essence of the covenant testifies to this fact. (52)On the very next page, Robertson argues that the covenants are one structurally and thematically; however, they are distinct in their administration. In other words, the essence of the covenants are indistinguishable, but God uses different mechanisms and structures to implement that covenant.
That argument reminds me of some words from one of my profs at Southeastern, which even now echo in my ear: "Covenant Theology is an amazing theological system. The only problem is, it has nothing whatsoever to do with the Bible." Though that may have been a bit of hyperbole, his conclusion is nowhere more clear to me than on this particular point that Robertson asserts. Robertson is helpful in many ways, particularly in demonstrating how Gentiles were incorporated into the Abrahamic Covenant from its inception. (Why should we be surprised if they are incorporated today via the Church?) But when I read Scripture, I find evidence that is incompatible with the notion that the biblical covenants are essentially the same.
We can begin at the text Robertson notes above: Jeremiah 31. Robertson argues that the "new covenant" is the same covenant as the covenant established through Moses at Sinai. But Jeremiah says this new covenant is not like that old covenant. He doesn't say the administration of the covenant changes. He says the covenant changes. The administration is part of the very essence of the new covenant. That's the argument Jeremiah makes in verses 33-34 when he says, "This is the covenant:" and then proceeds to tell us that part of the essential change in covenant is administration. Administration change is part and parcel to covenant change. Hebrews 8 quotes this passage and concludes (v 13), "In speaking of a new covenant, he makes the first one [covenant] obsolete."
We could walk through other texts in Hebrews, but let's instead back up to Galatians 3-4—a passage, incidentally, which presents no small problems for many Dispensationalists. Covenant Theologians will quite reasonably argue from 3:15-22 that the Sinai Covenant did not abrogate the earlier Abrahamic Covenant. But 4:21-31 explicitly declares that there are still two covenants (not two administrations of the same covenant). One of them—the covenant of Sinai—leads to slavery. The other—the covenant to which we are parties ("the Jerusalem above . . . is our mother") leads to freedom and inheritance of the promises.