While God’s mercies to the Israelites despite their disloyalty to the Sinaitic covenant are always justified on the basis of the Abrahamic promise, there are no passages that read, “Yet God remained faithful to David/the house of David for the sake of his covenant with Moses and the people at Horeb.” The covenant does not work in reverse. God never remains faithful to unfaithful national Israel on the basis of the Sinaitic covenant itself–for on that basis, as he repeatedly says, he would have scattered them long ago. And yet it is on the basis of the Sinaitic covenant that God exiles Judah and eventually, through Jesus’s prophetic ministry abolishes the theocracy and pronounces judgment upon it. This reiterates the fact that the ministry of Moses could not being about that blessedness that was the positive side of the sanctions–not because it was flawed, but because those who answered with one voice, “We will do all these things,” in fact did not.
– Michael S. Horton, God of Promise (Grand Rapids, MI: Baker Books) 99.