Objection 1. It would seem that it was not fitting for Christ to descend into hell, because Augustine says (Ep. ad Evod. cliv.): "Nor could I find anywhere in the Scriptures hell mentioned as something good." But Christ's soul did not descend into any evil place, for neither do the souls of the just. Therefore it does not seem fitting for Christ's soul to descend into hell.
Objection 2. Further, it cannot belong to Christ to descend into hell according to His Divine Nature, which is altogether immovable; but only according to His assumed nature. But that which Christ did or suffered in His assumed nature is ordained for man's salvation: and to secure this it does not seem necessary for Christ to descend into hell, since He delivered us from both guilt and penalty by His Passion which He endured in this world, as stated above (49, 1,3). Consequently, it was not fitting that Christ should descend into hell.
Objection 3. Further, by Christ's death His soul was separated from His body, and this was laid in the sepulchre, as stated above (Article 51). But it seems that He descended into hell, not according to His soul only, because seemingly the soul, being incorporeal, cannot be a subject of local motion; for this belongs to bodies, as is proved in Phys. vi, text. 32; while descent implies corporeal motion. Therefore it was not fitting for Christ to descend into hell.
On the contrary, It is said in the Creed: "He descended into hell": and the Apostle says (Ephesians 4:9): "Now that He ascended, what is it, but because He also descended first into the lower parts of the earth?" And a gloss adds: "that is--into hell."
I answer that It was fitting for Christ to descend into hell. First of all, because He came to bear our penalty in order to free us from penalty, according to Isaiah 53:4: "Surely He hath borne our infirmities and carried our sorrows." But through sin man had incurred not only the death of the body, but also descent into hell. Consequently since it was fitting for Christ to die in order to deliver us from death, so it was fitting for Him to descend into hell in order to deliver us also from going down into hell. Hence it is written (Hosea 13:14): "O death, I will be thy death; O hell, I will be thy bite." Secondly, because it was fitting when the devil was overthrown by the Passion that Christ should deliver the captives detained in hell, according to Zechariah 9:11: "Thou also by the blood of Thy Testament hast sent forth Thy prisoners out of the pit." And it is written (Colossians 2:15): "Despoiling the principalities and powers, He hath exposed them confidently." Thirdly, that as He showed forth His power on earth by living and dying, so also He might manifest it in hell, by visiting it and enlightening it. Accordingly it is written (Psalm 23:7): "Lift up your gates, O ye princes," which the gloss thus interprets: "that is--Ye princes of hell, take away your power, whereby hitherto you held men fast in hell"; and so "at the name of Jesus every knee should bow," not only "of them that are in heaven," but likewise "of them that are in hell," as is said in Philippians 2:10.
Reply to Objection 1. The name of hell stands for an evil of penalty, and not for an evil of guilt. Hence it was becoming that Christ should descend into hell, not as liable to punishment Himself, but to deliver them who were.
Reply to Objection 2. Christ's Passion was a kind of universal cause of men's salvation, both of the living and of the dead. But a general cause is applied to particular effects by means of something special. Hence, as the power of the Passion is applied to the living through the sacraments which make us like unto Christ's Passion, so likewise it is applied to the dead through His descent into hell. On which account it is written (Zechariah 9:11) that "He sent forth prisoners out of the pit, in the blood of His testament," that is, by the power of His Passion.
Reply to Objection 3. Christ's soul descended into hell not by the same kind of motion as that whereby bodies are moved, but by that kind whereby the angels are moved, as was said in I, 53, 1.
Source: Summa Theologica (New Advent)