One of the beautiful doctrines of the ancient church is the communicatio idiomatum, the teaching that everything about Christ’s divinity can be stated about his humanity and vice versa. It leads to startling statements like, ‘One of the Most Holy Trinity was crucified and died for us!’ Philosophically, it is a means of maintaining the unity of Christ in light of the fullness of his humanity and the fullness of his divinity.
The doctrine is important because of the fact that Jesus is affirmed by the Council of Chalcedon in 451 as possessing two natures but in a single person. This language of two natures is a fifth-century development, and it took a couple of centuries until St Maximus the Confessor (580-662) fleshed it out beautifully and magnificently after St Leo the Great’s use of such language in 448 had already rent the fabric of the church in two.
Nevertheless, there are hints of Leo’s insight already in the late fourth century. Thus St Gregory of Nazianzus (320-390):
Everything glorious in Scripture concerning the Son, you should apply to His Deity, that nature in Him which is non-physical, far above sufferings; everything lowly in Scripture concerning the Son, you should apply to His condition as the God who took our nature upon Him, humbling Himself for your sakes and was incarnate (we may as well sake ‘became Man’), and afterwards was glorified. (Third Theological Oration, 17, trans. Nick Needham, Daily Readings: The Early Church Fathers, 12 March)
St Gregory, however, is a bit subtler than Leo’s Tome. St Leo straightforwardly says that the humanity suffered, the divinity wrought miracles. St Gregory, on the other hand, posits everything about the humanity still to the divinity — in His incarnation as a human. And remember, St Gregory of Nazianzus is he who wrote, ‘What has not been assumed has not been saved,’ demonstrating that he believes in the fullness of Christ’s humanity.