My sources come from an old debate at Lane Keister’s blog. The main Reformed guy in the debate has since apostasized, so I am not using his name–and in any case, I “internalized” these sources and made them my own years ago. He merely cited the evidence. The finer points of the arguments are my own.
Christ is recognized as the mediator in his union with human nature; this does not mean he becomes the mediator via that union, for such would be Nestorianism or Adoptionism.
“it is nevertheless only in union with human nature that we recognize the person of the mediator.” (Christ and the Decree, by Richard Muller [The Labyrinth Press: Durham, North Carolina, 1986] pg. 29).
“Calvin does, in fact, speak of the ‘person of the mediator’ prior to the incarnation, in reference to the Old testament witness…The eternal Son is designated as mediator prior to the incarnation and performs his office in the communication of God’s Word to man.”(pg. 29).
This is why later Reformed would speak of the Covenant of Redemption–without the Covenant of Redemption, we are easy prey to heresy charges.
The “predestination of Christ,” such as it is, goes much deeper than a mere predestining of his human nature.
“It is not intended to intimate that Christ was possessed of a two-fold sonship, as he was divine and as he was human. Upon this point I must confer with Dr. Candlish in opposition to Dr. Crawford. His sonship is eternally one. Had he become the Son of God as human, and thus in addition to his divine sonship, assumed human sonship, the consequence would be involved that he became a human person, since sonship supposes personality. That doctrine the church has always rejected, The last attempt made to support it, by the school of the “Adoptionists” failed to receive the suffrages of the Roman Catholic Church, and has not been approved by the Protestant. …We are thus, if believers, first, made one with God’s Son by community of nature-we become his brethren and therefore sons of God with him. Secondly, we are partakers of his life, because partakers of his Spirit and are as he is in God the Father’s regard. Thirdly, we are possessed by imputation of filial obedience, which performed the condition upon which we are indefectibly instated as sons in the fatherly favor of God.”
John L. Girardeau, ed., George Blackburn, Discussions of Theological Questions (reprint,Harrisonburg, Va: Sprinkle Publications, 1886; Richmond: The Presbyterian Committee of Publication, 1905), 487-488. Footnote.
Girardeau anticipates the adoptionist charge and cuts it off at the pass.