St. Thomas seems to suggest that He would have died in this manner, perhaps from a heart attack at the age of 95: “Christ’s body was subject to the necessity of death and other like defects,” not because He contracted these defects through the manner of His birth, “as if taking them upon Himself as due to sin,” but because He assumed them voluntarily. The fundamental reason to think so is that “it was in order to satisfy for the sin of the human race that the Son of God, having taken flesh, came into the world. Now one satisfies for another’s sin by taking on himself the punishment due to the sin of the other. But these bodily defects, to wit, death, hunger, thirst, and the like, are the punishment of sin, which was brought into the world by Adam… Hence it was useful for the end of Incarnation that He should assume these penalties in our flesh and in our stead…” (ST, III, 14)
But I reject the premise of this Pauline teaching that the Father punished the Son as some sort of hated scapegoat in order for the human race to evade divine justice. Without this disgraceful and absurd idea, does the argument still hold up?
There are two reasons to think that Christ would have died of old age. First, because of the easy syllogism:
(1) All men are mortal.
(2) Christ was a man.
(3) Christ was mortal.
If Jesus’ body was naturally immortal and would not age beyond 30 years old or whatever, then His body had to have worked very differently from our own bodies. It would then be difficult to maintain that Christ took on human nature.
Second, because of the realistic consideration to the effect that “what good would it have done Him?” Jesus knew that He would die on the cross, unjustly destroyed by those He loved, and an ageless body would have been entirely useless to Him.
There are, however, two other possibilities. First, St. Thomas argues that Adam in his Garden in the state of original righteousness and pure uncorrupt nature was graced with natural immortality and the “fullness of health, i.e., vigor of incorruptibility.” Upon his sin, this grace was withdrawn, and Adam, now expelled into the world, ended up growing old and dying as per the functioning of human nature as we now know it. Could it be that, say, at Jesus’ baptism, the Holy Spirit similarly imparted immortality upon His body: “heaven was opened, and he saw the Spirit of God descending like a dove and alighting on him”? (Mt 3:16)
Unfortunately, there is no Scriptural support whatsoever for this opinion.
Second, it may be that Jesus’ bodily immortality resulted from the union of His human nature with His divine nature or Godhead; or from the eminence or even glory that accrued to His human soul. St. Thomas admits that this is possible but denies that it was actual: “the beatitude remained in the soul, and did not flow into the body; but the flesh suffered what belongs to a passible nature; thus Damascene says… that, ‘it was by the consent of the Divine will that the flesh was allowed to suffer and do what belonged to it.'”
St. Thomas lists two more reasons “for the body assumed by the Son of God to be subject to human infirmities and defects” that do not depend on the dubious theory of “punishment”:
1) In order to cause belief in Incarnation. For since human nature is known to men only as it is subject to these defects, if the Son of God had assumed human nature without these defects, He would not have seemed to be true man, nor to have true, but imaginary, flesh, as the Manicheans held.
2) In order to show us an example of patience by valiantly bearing up against human passibility and defects. Hence it is said… that He “endured such opposition from sinners against Himself, that you be not wearied, fainting in your minds.”
On the whole, it seems somewhat more plausible that Jesus would indeed have died of old age, perhaps in the manner of the rather honorable Moses’ death, in His pre-resurrection body, had He not been murdered by us in His prime.