Therefore, following the holy Fathers, we all with one accord teach men to acknowledge one and the same Son, our Lord Jesus Christ, at once complete in Godhead and complete in manhood; truly God and truly man, consisting also of a reasonable soul and body; of one substance with the Father as regards his Godhead, and at the same time of one substance with us as regard his manhood; like us in all respects, apart from sin;
. . .
one and the same Christ, Son, Lord, Only-begotten, recognized in two natures, without confusion, without change, without division, without separation. . . .
A few days ago, I was reciting the Chalcedonian Definition (451), as I generally do on Tuesday, Thursday and Saturday mornings, and my mind went to my own constitution and that of all human beings. What, I wondered, are the implications of the incarnate Son’s likeness with us “in all respects,” for the human body and soul?
The one person had the essential attributes of both divine and human nature, including a human body, and the one person also had a human soul, according to Chalcedon. I have read numerous presentations of the biblical case for physicalism. Evangelicals who take this position usually describe this as a “non-reductive” physical, but they still deny that humans are any more (substantially) than their material aspect. In spite of those arguments, however, I remain convinced that the very possibility of Christ’s consubstantiality with us entails that we are 2 substances, otherwise Jesus was a “one-of-a-kind” human being, the only one who had two substantial aspects.
Regarding the nature of the human soul, it is unlike the souls that enliven many other living creatures because it is personal, and our personal being exists and persists principally in the soul. It is because we have human souls that we develop human bodies. Consequently, in taking upon himself human being (becoming a man), the Word can not have added a human soul to his divine being, which was purely soul, or he’d be two persons. So, in adding human attributes to his divine nature in one person, the personhood of the incarnate Word (who was named Jesus) inhered in a soul that was then both divine and human.
The importance of our conception of Christ’s own constituent nature (as just like ours), is seen most clearly when we think about his death. It was Jesus’ consubstantiality that kept him alive as the one person, both divine and human, during the intermediate state. If we, and therefore Jesus (as one who shared our humanity in all respects expect for sin), were only of one substance (material), as unitive substantialists propose, his death would have entailed either temporary dissolving of the incarnation (destruction of his humanity) or the temporary personal annihilation of the eternal Word.
In short, both the genuineness of Christ’s persistent humanness and the inextinguishability of his divine personhood indicate that humans are constituted of two substances, body and soul/spirit. It is because the Word continued his endless existence as God, who alone has immortality inherently, but did not stop being human when his body was killed, that we know that we too will continue to exist personally, though purely human, when we die.