From the pen of Pascal Denault:
What I mean is this: The law, introduced 430 years later, does not set aside the covenant previously established by God and thus do away with the promise. For if the inheritance depends on the law, then it no longer depends on the promise; but God in his grace gave it to Abraham through a promise. (Gal. 3:17-18)
Paul clearly affirms that it is through the Abrahamic Covenant that God promised his grace and that the Mosaic Covenant which came about 430 years later did not bring the inheritance nor did it replace the Abrahamic Covenant . The paeodbaptists understood from this passage that the Abrahamic Covenant was the Covenant of Grace, the covenant through which God grants his grace to Abraham and his posterity, and that the Judaizers were mistaken in demanding obedience to the Law of Moses as a condition in order to obtain the inhereitance. The Presbyterian paradigm of the Covenant of Grace was confirmed by this interpretation: the Covenant of Grace that God concluded with Abraham included his physical posterity; the Covenant of Grace was, therefore, a Covenant of a mixed nature in which one entered at birth.
The Baptists were partially in agreement with the Presbyterian interpretation of this passage. They recognized that Paul exposed the legalistic error of the Judaizers by basing his argument on the fact the promised inheritance was only granted by the grace of God and that this grace was given to Abraham when God made a covenant with him; the Law which had come into existence 430 years before did not replace grace as a method to obtain the inheritance. However, the Baptists did not support the Presbyterian paradigm of the Covenant of Grace which wanted the latter to include the physical posterity of Abraham and of believers. Instead they applied their own paradigm of the Covenant of Grace (revealed/concluded) to this passage: the Covenant of Grace was revealed to Abraham, but the formal covenant that God concluded with him was not the Covenant of Grace. What is more Galatians 3:17-18 does not affirm that God gave his grace to Abraham through the covenant, but through the promise. In other words, the Abrahamic Covenant contained a promise; this promise was the revelation of the covenant of grace. The Abrahamic Covenant did include the physical posterity of Abraham, but it was not in the Covenant of Grace even if it was in a covenant that revealed the grace of God by way of promise.
…The Scriptures mention several dual principles. Regarding the covenant concluded with Abraham, the Scriptures also present a dualism. Abraham possessed a physical posterity as well as a spiritual posterity (Rom. 9:6-8; Gal. 4:22-31); there was an external circumcision of the flesh and an internal circumcision of the heart (Rom. 2:28-29); there was a promised land here on earth and a heavenly kingdom (Heb. 11:8-10). The Baptist pastor Hercules Collins taught this doctrine in his catechism. “We must know the Covenant made with Abraham had two parts: first, a spiritual, which consisted in God’s promising to be a God to Abraham, and all his Spiritual-Seed in a peculiar manner…”
The paedobaptists and the Baptists mutually recognized this dualism, but in a completely different way. The paedobaptists considered this dualism within one covenant. According to them, this covenant included a physical reality, external and earthly, combined with a spiritual reality, internal and celestial, exactly as in their understanding of the Covenant of Grace wherein there was an internal substance and an external administration. The paedobaptists made a distinction between these two realities but they refused to separate them in two distinct covenants. John Ball applied this paradigm in order to unite the physical and spiritual aspects of the Abrahamic Covenant: “And therein the inward force and vertue of the [Abrahamic] Covenant is to be distinguished from the outward administration.” Although they recognized that the posterity of Abraham was both physical and spiritual at the same time , the paedobaptists refused to see two posterities, because, according to them, Abraham had only one posterity mad up of the mixed people of the Covenant of Grace.This point was crucial, because if Abraham had two distinct posterities, the Baptists were right not to mix the natural (unregenerate) posterity and the spiritual (regenerate) posterity of Abraham. Inversely, if Abraham had only one mixed posterity, the paedobaptists were right to include those who were saved and those who were not saved in the Covenant of Grace. Samuel Petto, a paedobaptist, had understood this critical concern:
“Hence see the true meaning of Gal. 3.16. To Abraham and his seed were the Promises made: he saith not unto seeds, as of many, but as of one, and to thy seed, which is Christ: i.e. Always Abraham had but one seed, Christ, and those that are Christ’s, and are of the Faith as to Justification, he never had two seeds for that end; in the times of the Old Testament there was but one seed, not two seeds, one by the Law, and another by Promise, but only one in Christ by Promise…”
“And so it is not in the least mentioned to exclude Infants, as a fleshly seed, from an ecclesiastical seed, nor to repeal any priviledge or limit to cut them off from what they had before the coming of Christ…”
The paedobaptists refused to separate the dualities of the Abrahamic Covenant in order to preserve their model of the Covenant of Grace which integrated them. The Covenant of Grace, to include children, had to include both earthly and heavenly realities at the same time. Baptist theologians understood that if they kept these dualities united in the same covenant, they no longer had any reason to reject the paedobaptist model of the covenant of grace. In fact, if the Covenant of Grace revealed to Abraham included both his physical and spiritual posterity at once, why would it have been otherwise under the New Covenant? Therefore, not only did Particular Baptist theology make the distinction between the physical and spiritual posterities of Abraham, but it also strictly separated them into two separate categories. The Baptists saw two posterities in Abraham, two inheritances and consequently two covenants.
Galatians 4:22-31 constitutes a key passage of Particular Baptist federalism. In it we read:
For it is written that Abraham had two sons, one by the slave woman and the other by the free woman. His son by the slave woman was born according to the flesh, but his son by the free woman was born as the result of a divine promise. These things are being taken figuratively: The women represent two covenants. One covenant is from Mount Sinai and bears children who are to be slaves: This is Hagar. Now Hagar stands for Mount Sinai in Arabia and corresponds to the present city of Jerusalem, because she is in slavery with her children.But the Jerusalem that is above is free, and she is our mother. For it is written: “Be glad, barren woman, you who never bore a child; shout for joy and cry aloud, you who were never in labor; because more are the children of the desolate woman than of her who has a husband.” Now you, brothers and sisters, like Isaac, are children of promise. At that time the son born according to the flesh persecuted the son born by the power of the Spirit. It is the same now. 30But what does Scripture say? “Get rid of the slave woman and her son, for the slave woman’s son will never share in the inheritance with the free woman’s son.” Therefore, brothers and sisters, we are not children of the slave woman, but of the free woman.
From this passage, Nehemiah Coxe understood, not that the posterity of Abraham was of a mixed nature, but that Abraham had two distinct posterities and that it was necessary to determine the inheritance of each of these posterities on the basis of their respective promises. He writes:
“Abraham is to be considered in a double capacity: he is the father of all true believers and the father and root of the Israelite nation. God entered into covenant with him for both of these seeds and since they are formally distinguished from one another, their covenant interest must necessarily be different and fall under a distinct consideration. The blessings appropriate to either must be conveyed in a way agreeable to their peculiar and respective covenant interest. And these things may not be confounded without a manifest hazard to the most important articles in the Christian religion.”
We have here a fundamental difference with the paedobaptist understanding of the Abrahamic Covenant. The aforementioned placed the natural and spiritual posterities of Abraham in the same Covenant of Grace; the first inheriting only physical blessings of the covenant and the second benefiting also from the spiritual privileges. Coxe affirmed that it was not so. This is why he declares that since the Scriptures formally distinguished between the two posterities, one could not mix both up under the same covenant without compromising important doctrines.
This understanding was vigorously affirmed amongst all Particular Baptist theologians and characterized their federalism from its origin. Spilsbury writes, “There was in Abraham at that time a spirituall seed and a fleshly seed. Between which seeds God ever distinguished through all their Generations.” *
* Pascal Denault, The Distinctiveness of Baptist Covenant Theology (Birmingham: Solid Ground Christian Books, 2013), p. 115-120