Objection 1. It would seem that there was a severance in death between Christ's Godhead and His soul, because our Lord said (John 10:18): "No man taketh away My soul from Me: but I lay it down of Myself, and I have power to lay it down, and I have power to take it up again." But it does not appear that the body can set the soul aside, by separating the soul from itself, because the soul is not subject to the power of the body, but rather conversely: and so it appears that it belongs to Christ, as the Word of God, to lay down His soul: but this is to separate it from Himself. Consequently, by death His soul was severed from the Godhead.
Objection 2. Further, Athanasius [Vigilius Tapsensis, De Trin. vi; Bardenhewer assigns it to St. Athanasius: 45, iii. The full title is De Trinitate et Spiritu Sancto] says that he "is accursed who does not confess that the entire man, whom the Son of God took to Himself, after being assumed once more or delivered by Him, rose again from the dead on the third day." But the entire man could not be assumed again, unless the entire man was at one time separated from the Word of God: and the entire man is made of soul and body. Therefore there was a separation made at one time of the Godhead from both the body and the soul.
Objection 3. Further, the Son of God is truly styled a man because of the union with the entire man. If then, when the union of the soul with the body was dissolved by death, the Word of God continued united with the soul, it would follow that the Son of God could be truly called a soul. But this is false, because since the soul is the form of the body, it would result in the Word of God being the form of the body; which is impossible. Therefore, in death the soul of Christ was separated from the Word of God.
Objection 4. Further, the separated soul and body are not one hypostasis, but two. Therefore, if the Word of God remained united with Christ's soul and body, then, when they were severed by Christ's death, it seems to follow that the Word of God was two hypostases during such time as Christ was dead; which cannot be admitted. Therefore after Christ's death His soul did not continue to be united with the Word.
On the contrary, Damascene says (De Fide Orth. iii): "Although Christ died as man, and His holy soul was separated from His spotless body, nevertheless His Godhead remained unseparated from both--from the soul, I mean, and from the body."
I answer that, The soul is united with the Word of God more immediately and more primarily than the body is, because it is through the soul that the body is united with the Word of God, as stated above (Question 6, Article 1). Since, then, the Word of God was not separated from the body at Christ's death, much less was He separated from the soul. Accordingly, since what regards the body severed from the soul is affirmed of the Son of God--namely, that "it was buried"--so is it said of Him in the Creed that "He descended into hell," because His soul when separated from the body did go down into hell.
Reply to Objection 1. Augustine (Tract. xlvii in Joan.), in commenting on the text of John, asks, since Christ is Word and soul and body, "whether He putteth down His soul, for that He is the Word? Or, for that He is a soul?" Or, again, "for that He is flesh?" And he says that, "should we say that the Word of God laid down His soul"... it would follow that "there was a time when that soul was severed from the Word"--which is untrue. "For death severed the body and soul... but that the soul was severed from the Word I do not affirm... But should we say that the soul laid itself down," it follows "that it is severed from itself: which is most absurd." It remains, therefore, that "the flesh itself layeth down its soul and taketh it again, not by its own power, but by the power of the Word dwelling in the flesh": because, as stated above (Article 2), the Godhead of the Word was not severed from the flesh in death.
Reply to Objection 2. In those words Athanasius never meant to say that the whole man was reassumed--that is, as to all his parts--as if the Word of God had laid aside the parts of human nature by His death; but that the totality of the assumed nature was restored once more in the resurrection by the resumed union of soul and body.
Reply to Objection 2. Through being united to human nature, the Word of God is not on that account called human nature: but He is called a man--that is, one having human nature. Now the soul and the body are essential parts of human nature. Hence it does not follow that the Word is a soul or a body through being united with both, but that He is one possessing a soul or a body.
Reply to Objection 4. As Damascene says (De Fide Orth. iii): "In Christ's death the soul was separated from the flesh: not one hypostasis divided into two: because both soul and body in the same respect had their existence from the beginning in the hypostasis of the Word; and in death, though severed from one another, each one continued to have the one same hypostasis of the Word. Wherefore the one hypostasis of the Word was the hypostasis of the Word, of the soul, and of the body. For neither soul nor body ever had an hypostasis of its own, besides the hypostasis of the Word: for there was always one hypostasis of the Word, and never two."
Source: Summa Theologica (New Advent)