14 He Was Crucified Under Pontius Pilate and Was Buried: He Descended into Hell. The Apostle Paul teaches us that we ought to have “the eyes of our understanding enlightened” “that we may understand what is the height and breadth and depth.” “The height and breadth and depth” is a description of the Cross, of which that part which is fixed in the earth he calls the depth, the height that which is erected upon the earth and reaches upward, the breadth that which is spread out to the right hand and to the left.
Since, therefore, there are so many kinds of death by which it is given to men to depart this life, why does the Apostle wish us to have our understanding enlightened so as to know the reason why, of all of them, the Cross was chosen in preference for the death of the Saviour. We must know, then, that that Cross was a triumph. It was a signal trophy. A triumph is a token of victory over an enemy. Since then Christ, when He came, brought three kingdoms at once into subjection under His sway (for this He signifies when he says, “That in the name of Jesus every knee should bow, of things in heaven, and things on earth, and things under the earth”), and conquered all of these by His death, a death was sought answerable to the mystery, so that being lifted up in the air, and subduing the powers of the air, He might make a display of His victory over these supernatural and celestial powers.
Moreover the holy Prophet says that “all the day long He stretched out His hands” to the people on the earth, that He might both make protestation to unbelievers and invite believers: finally, by that part which is sunk under the earth, He signified His bringing into subjection to Himself the kingdoms of the nether world.
Source: Commentary on the Apostles' Creed (New Advent)