Clearly the covenant of grace displays organic continuity and perpetuity. Wicked men like Cain are Eve's physical children, but they are not her seed. They are neither partakers of this promise, nor parties of the covenant of grace. [Greg Nichols, Covenant Theology: A Reformed and Baptistic Perspective on God's Covenants (Vestavia Hills, AL: Solid Ground Christian Books, 2011), 126]
Cain, Abel and Seth were all Eve's physical children... Abel and Seth were her spiritual children. Cain was the devil's spiritual child. (Ibid., 127)
Greg Nichols' book has been an interesting book from a Reformed Baptist perspective. It is interesting to note how Reformed Baptist Covenant theology is done, and how it differs from traditional Reformed Covenant Theology.
It is slightly disturbing to note in our excerpts that a Reformed Baptist thinks that the federal representation in the Covenant of Grace has to do with who is or isn't Eve's seed. We note that the proto-evangel in Genesis 3:15 concerns Eve's seed (singular) and the serpent's seed (also singular). In other words, this speaks about Jesus Christ, born of a woman, against Satan and his "seed" (the Antichrist). Therefore, to deliberate whether Cain or Abel is or is not Eve's seed is in error, because Eve is not the federal representative of the Covenant of Grace. Christ rather is the federal representative, the Mediator and Surety, of the Covenant of Grace.
The struggle between the seed of the women and the seed of the serpent is the struggle between Christ and the Devil, and thus more broadly of those who follow Christ and those who follow the Devil. Cain manifests himself to be the serpent's seed because he does the works of the devil, which has nothing to do with his parentage. His parentage is of Adam and thus of the broken Covenant of Works, not of the Covenant of Grace. Conversely, Abel and Seth are not Eve's spiritual children, but rather they have Christ as their federal head.
This strange contorted view of the first family fuels Nichols' view of trying to make the Covenant of Grace purely spiritual and not physical. Using this strange view concerning Eve's children, the point is made that it is Eve's spiritual children that are part of the Covenant of Grace, not Eve's physical children. But if we perceive the covenantal representation correctly, then we see that the it is not about whether one's children is "spiritual" that determines whether one is in the Covenant of Grace, but rather whether said children have their federal representation in Christ. Abel and Seth have their federal head in Christ, and are thus part of the Covenant of Grace. Cain has only his federal head in Adam, and thus he dies in his sins.