The doctrine known as the extra Calvinisticum states that the Son of God is not limited to nor circumscribed by his human nature. Even “after” the incarnation, the eternal Son still continues to exist as God, upholding the universe by the Word of his power, along with the Father and the Spirit. The doctrine emerged out the Reformation controversies over the presence of Christ in the Lord’s Supper. The Lutherans maintained that the Son’s human nature became ubiquitous by virtue of its union with the divine nature and could therefore be present “in, with, and under” the elements of the Eucharist (a misapplication of the ancient doctrine of the communicatio idiomatum, or communication of attributes). The logic of the Lutherans seemed to be that wherever the divinity of Christ is, there also is the humanity of Christ. The Calvinists, on the other hand, maintained that the incarnation does not introduce a change or mutation to either the human or the divine nature of Christ; instead, since both natures retain their integrity, the Son must continue to exist in his divinity, even apart from his human nature. The Lutherans lampooned this view, labeling it Calvin’s extra, meaning, “outside of”–as if Christ’s divinity was sort of spilling out of him in a spatial sense (for more on the extra, see Paul Helm’s essay on it here).
But the Calvinists had the upper hand in this debate–in terms of both Scripture and the Christian tradition. On the latter, David Willis has argued convincingly that the Calvinist view is so well attested in the tradition that the doctrine is more properly termed the extra Catholicum or the extra Patristicum, rather than the extra Calvinisticum.
One of the most common objections to the extra is that it runs the risk of Nestorianism. If Christ has a divine life, so to speak, outside of his incarnate experience, then doesn’t this entail that he is functioning effectively as two persons, one divine and one human? Gerald Hawthorne, for example, argues that the early church fathers implicitly taught this view, namely, that Christ has a kind of “dual existence” as God and as man. According to Hawthorne, this view either risks Nestorianism or else a kind of “de facto Docet[ism], failing to estimate fully the humanity in which divinity made itself visible.”
Ironically, Hawthorne cites Cyril of Alexandria as an example of this view. I say “ironically,” because Cyril was, of course, the chief anti-Nestorian in the lead up to the Third Ecumenical Council, the Council of Ephesus (431), which denounced the two-persons heresy. So if Cyril himself affirms the extra, then it seems highly unlikely that it actually risks the error of Nestorianism. Here is the relevant portion on the issue from Cyril’s Third Epistle to Nestorius:
And we do not say that the flesh was changed into the Godhead, or again that the ineffable nature of God the Word was perverted into that of the flesh, for He is immutable and unalterable, ever abiding the same, according to the Scriptures; but while visible as a babe in swaddling clothes and yet in the bosom of the Virgin who bare him, He was filling all creation as God, and was enthroned with Him who begat Him. For the divinity is immeasurable and without magnitude, nor does it admit of circumspection.
The Word is united to flesh and so Christ is one person, but this does not mean that his divinity is contracted, so to speak, to his human nature. He remains the Word even in his incarnate state. Thus, because he has two natures, there is nothing improper in ascribing to him, as a single person, attributes and activities distinctive of his two natures: nursing at Mary’s breast, while at the same time filling all creation. So perhaps we could, with some justification, even speak of the doctrine as the extra Cyrillicum.