Aquinas’ first argument in favor of Scotus’ position, and his response to the contrary:
Objection 1. It would seem that if man had not sinned, God would still have become incarnate. For the cause remaining, the effect also remains. But as Augustine says (De Trin. xiii, 17): “Many other things are to be considered in Incarnation of Christ besides absolution from sin”; and these were discussed above (Article 2). Therefore if man had not sinned, God would have become incarnate. (Summa P.III, Q.1, Art.3).
And his response:
Reply to Objection 1. All the other causes which are assigned in the preceding article have to do with a remedy for sin. For if man had not sinned, he would have been endowed with the light of Divine wisdom, and would have been perfected by God with the righteousness of justice in order to know and carry out everything needful. But because man, on deserting God, had stooped to corporeal things, it was necessary that God should take flesh, and by corporeal things should afford him the remedy of salvation. Hence, on John 1:14, “And the Word was made flesh,” St. Augustine says (Tract. ii): “Flesh had blinded thee, flesh heals thee; for Christ came and overthrew the vices of the flesh.”
In substance, and this is why the discussion is so important, if we say with the Angelic Doctor that had Adam not sinned the Word would not have become incarnate, then this reduces the mediation of Christ to a conditional one. The Mediator exists as a reparation; the Mediator is sent to redeem and His mediation is but a part of man’s Redemption. Hence Thomas’ statement: “For if man had not sinned, he would have been endowed with the light of Divine wisdom, and would have been perfected by God with the righteousness of justice in order to know and carry out everything needful.” In other words, Christ’s mediation would have been superfluous if Adam had not sinned and man would have been in friendship with God directly without any Mediator. From the thomistic perspective, Christ’s mediation was only a remedy to sin after man lost that “light of Divine wisdom” and the mediation of Jesus was not central to God’s plan, but a sort of “plan B” because Adam and Eve would rupture their relationship with God.
We do well to note here that for Scotus and the Franciscan school Christ’s mediation (and consequently the subordinate mediation of His Mother) is unconditional, absolute. When God predestined us and the Angels, it was in Christ “before the foundation of the world” (Eph 1:4). As such, Redemption is a form (quintessential for us after the fall!) of His mediation. As such, Christ is Mediator and King to the Angels, not because of the sin of Adam, but simply because God willed it that way – Christ was predestined to be the Head of the whole Church, of all the elect – angelic and human, quite apart from any consideration of man’s fall. God always willed to mediate His grace and glory through the union of the Word with the Sacred Humanity of Christ and His headship extends from Adam and Eve before the fall, to the Angels, to all the elect because they are predestined to grace and glory in Him.
Fr. Maximilian M. Dean