Like all medievals, like all scholastics, Thomas asks unusual questions. Nearly always they are odd questions that help you turn corners to see roads you wouldn’t have seen otherwise.
He asks, “Was the Godhead of the Son separated from the flesh of Christ when he died?” ( ST III, 50, 2), and says, No. It’s there in the creed too: The Son was conceived, suffered, died, buried. Besides, the human nature of the Son exists only by virtue of the Son’s assumption, and if the assumption ceased, so would the flesh. At the same time (III, 50, 3), the Word remained joined to the soul of Jesus. He cites John Damascene: “His Godhead remained unseparated from both - from the soul, I mean, and from the body.”
He also asks (III, 50, 4) whether Christ was a man during the three days after His death, and answers No:
“it belongs to the truth of the death of man or animal that by death the subject ceases to be man or animal.” Without the soul that is the formal cause of a man, the man simply isn’t. Christ was a “dead man,” but not, simpliciter , “man.” It’s a stark point: We take death too lightly. We let ourselves think that he or she is still there. Aquinas would agree, of course, that the soul continues to exist, but he would want us to feel the abyss of death: Once there was a man, now there isn’t.
But the cash value of this odd series of questions comes when he points out that Christ’s death is unique: “The dead body of everyone else does not continue united to an abiding hypostasis, as Christ’s dead body did” (III, 50, 5). When we die, on Thomas’s account, body and soul are separated, and the soul “experiences” the separation. But the soul doesn’t “experience” the death of the body. The soul departs and leaves the body. With Christ, though, a third something remains with body and soul in their separation, that third something being the eternal Son of God.
He is not just a body suffering the soul’s departure, or the soul losing its body. He remains with the body in the grave, something a human soul has never done. He remains with the bodiless soul, something no human body has ever done. He suffers the body’s loss of soul, and the soul’s loss of body. The Word remains with body and soul, stretched out between the grave and wherever it is the soul travels (Thomas thinks Christ’s soul descended to hell! III, 49, 6).
No human ever tasted death as fully as He. The Word doesn’t just experience death. Joined to body and soul, He passes through most deathly death, takes it all, for us.