Objection 1. It would seem that Christ was a man during the three days of His death, because Augustine says (De Trin. ii.): "Such was the assuming [of nature] as to make God to be man, and man to be God." But this assuming [of nature] did not cease at Christ's death. Therefore it seems that He did not cease to be a man in consequence of death.
Objection 2. Further, the Philosopher says (Ethic. ix) that "each man is his intellect"; consequently, when we address the soul of Peter after his death we say: "Saint Peter, pray for us." But the Son of God after death was not separated from His intellectual soul. Therefore, during those three days the Son of God was a man.
Objection 3. Further, every priest is a man. But during those three days of death Christ was a priest: otherwise what is said in Psalm 109:4 would not be true: "Thou art a priest for ever." Therefore Christ was a man during those three days.
On the contrary, When the higher [species] is removed, so is the lower. But the living or animated being is a higher species than animal and man, because an animal is a sensible animated substance. Now during those three days of death Christ's body was not living or animated. Therefore He was not a man.
I answer that, It is an article of faith that Christ was truly dead: hence it is an error against faith to assert anything whereby the truth of Christ's death is destroyed. Accordingly it is said in the Synodal epistle of Cyril [Act. Conc. Ephes. P. I, cap. xxvi]: "If any man does not acknowledge that the Word of God suffered in the flesh, and was crucified in the flesh and tasted death in the flesh, let him be anathema." Now it belongs to the truth of the death of man or animal that by death the subject ceases to be man or animal; because the death of the man or animal results from the separation of the soul, which is the formal complement of the man or animal. Consequently, to say that Christ was a man during the three days of His death simply and without qualification, is erroneous. Yet it can be said that He was "a dead man" during those three days.
However, some writers have contended that Christ was a man during those three days, uttering words which are indeed erroneous, yet without intent of error in faith: as Hugh of Saint Victor, who (De Sacram. ii) contended that Christ, during the three days that followed His death, was a man, because he held that the soul is a man: but this is false, as was shown in I, 75, 4. Likewise the Master of the Sentences (iii, D, 22) held Christ to be a man during the three days of His death for quite another reason. For he believed the union of soul and flesh not to be essential to a man, and that for anything to be a man it suffices if it have a soul and body, whether united or separated: and that this is likewise false is clear both from what has been said in I, 75, 4, and from what has been said above regarding the mode of union (2, 5).
Reply to Objection 1. The Word of God assumed a united soul and body: and the result of this assumption was that God is man, and man is God. But this assumption did not cease by the separation of the Word from the soul or from the flesh; yet the union of soul and flesh ceased.
Reply to Objection 2. Man is said to be his own intellect, not because the intellect is the entire man, but because the intellect is the chief part of man, in which man's whole disposition lies virtually; just as the ruler of the city may be called the whole city, since its entire disposal is vested in him.
Reply to Objection 3. That a man is competent to be a priest is by reason of the soul, which is the subject of the character of order: hence a man does not lose his priestly order by death, and much less does Christ, who is the fount of the entire priesthood.
Source: Summa Theologica (New Advent)