The Old Covenant was not added to the Abrahamic Covenant-it was an additional covenant. The Abrahamic Covenant was one covenant, and the Old Covenant was a separate and additional covenant of its own.
These two covenants stand in relation to each other much as the special Sabbath Covenant (Exodus 31:12-17, showing it to be a separate covenant) stands in relation to the Old Covenant. The Sabbath Covenant establishes that the Sabbath is the sign of God's people. It is a separate covenant in addition to the Old Covenant.
This verse explains why the Old Covenant is not added to the Abrahamic Covenant. One cannot add to a contract that has already been signed, sealed, and delivered.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part Twenty-Seven)
Four hundred and thirty years after the covenant God made with Abraham the law came. We understand that it was already in existence, but it was given to Israel in a codified form as a portion of the covenant that God made with him.
The real beginning of the Old Testament church was not at Mt. Sinai but in the land of inheritance where Abraham pitched his tent 430 years earlier; the Old Testament church began with Abraham. And the New Testament church, in that sense, also began at the same time—because Abraham is the father of the faithful. This highlights how important Nisan 15 is.
We understand that the real, true beginning of the New Testament church was on the day of Pentecost—when God gave His Spirit. But these are the very roots of that event! By combining Exodus 12:40-41 with Genesis 17 and Galatians 3, we know that these two events, God's covenant with Abraham and Israel's exodus from Egypt, took place on the same date 430 years apart.
From that small beginning with Abraham and Sarah came Isaac and Rebecca and then Jacob and his wives and children. Joseph was sold as a slave into Egypt. Then the famine drove Jacob down into Egypt along with all of his relatives, where they grew into a sizable nation subjugated by the Egyptians. They became a nation of about 2½ million people. Then came the raising up of Moses and the destruction of Egypt culminating in the slaying of the firstborn on Nisan 14. And then the climax: The children of Israel leaving Egypt 430 years to the day that God entered into the covenant with Abraham!
On that very day, Abraham, Ishmael, and all the males of Abraham's household were circumcised, and thus they received the sign of the covenant. The covenant made at Mt. Sinai was essentially the same covenant as that entered into by God and Abraham but expanded to include the entire nation (that is, all the descendants of Abraham). Added to it, then, were civil and ceremonial laws necessary for administering the covenant to the whole nation. That makes Nisan 15 a very significant date.
John W. Ritenbaugh
The Night to be Much Observed