I answer that, The expression "simply" can be taken in two senses. In the first instance by taking "simply" to be the same as "absolutely"; thus "that is said simply which is said without addition," as the Philosopher put it (Topic. ii): and in this way the dead and living body of Christ was simply identically the same: since a thing is said to be "simply" identically the same from the identity of the subject. But Christ's body living and dead was identical in its suppositum because alive and dead it had none other besides the Word of God, as was stated above (A. 2). And it is in this sense that Athanasius is speaking in the passage quoted.
In another way "simply" is the same as "altogether" or "totally": in which sense the body of Christ, dead and alive, was not "simply" the same identically, because it was not "totally" the same, since life is of the essence of a living body; for it is an essential and not an accidental predicate: hence it follows that a body which ceases to be living does not remain totally the same. Moreover, if it were to be said that Christ's dead body did continue "totally" the same, it would follow that it was not corrupted—I mean, by the corruption of death: which is the heresy of the Gaianites, as Isidore says (Etym. viii), and is to be found in the Decretals (xxiv, qu. iii). And Damascene says (De Fide Orth. iii) that "the term 'corruption' denotes two things: in one way it is the separation of the soul from the body and other things of the sort; in another way, the complete dissolving into elements. Consequently it is impious to say with Julian and Gaian that the Lord's body was incorruptible after the first manner of corruption before the resurrection: because Christ's body would not be consubstantial with us, nor truly dead, nor would we have been saved in very truth. But in the second way Christ's body was incorrupt."
Reply Obj. 1: The dead body of everyone else does not continue united to an abiding hypostasis, as Christ's dead body did; consequently the dead body of everyone else is not the same "simply," but only in some respect: because it is the same as to its matter, but not the same as to its form. But Christ's body remains the same simply, on account of the identity of the suppositum, as stated above.
Reply Obj. 2: Since a thing is said to be the same identically according to suppositum, but the same specifically according to form: wherever the suppositum subsists in only one nature, it follows of necessity that when the unity of species is taken away the unity of identity is also taken away. But the hypostasis of the Word of God subsists in two natures; and consequently, although in others the body does not remain the same according to the species of human nature, still it continues identically the same in Christ according to the suppositum of the Word of God.
Reply Obj. 3: Corruption and death do not belong to Christ by reason of the suppositum, from which suppositum follows the unity of identity; but by reason of the human nature, according to which is found the difference of death and of life in Christ's body. _______________________
SIXTH ARTICLE [III, Q. 50, Art. 6]
Whether Christ's Death Conduced in Any Way to Our Salvation?
Objection 1: It would seem that Christ's death did not conduce in any way to our salvation. For death is a sort of privation, since it is the privation of life. But privation has not any power of activity, because it is nothing positive. Therefore it could not work anything for our salvation.
Obj. 2: Further, Christ's Passion wrought our salvation by way of merit. But Christ's death could not operate in this way, because in death the body is separated from the soul, which is the principle of meriting. Consequently, Christ's death did not accomplish anything towards our salvation.
Obj. 3: Further, what is corporeal is not the cause of what is spiritual. But Christ's death was corporeal. Therefore it could not be the cause of our salvation, which is something spiritual.