How do we know Jesus went to Hell to bring his followers up to heaven?
There is a passage where Christ describes hell/hades as containing two sections, torture for the wicked and paradise for the righteous in his parable about Lazarus and the rich man. In the Apostles’ Creed we say that Christ “descended into hell; on the third day he rose again from the dead”. Christ was with the good thief in the paradise section.
Christ descended into hell (the paradise part) and set the Old Testament saints free:
Ephesians 4:8-10 Therefore it is said, “When he ascended on high he led a host of captives, and he gave gifts to men.” (In saying, “He ascended,” what does it mean but that he had also descended into the lower parts of the earth? He who descended is he who also ascended far above all the heavens, that he might fill all things.)
Catechism of the Catholic Church
633 Scripture calls the abode of the dead, to which the dead Christ went down, “hell” — Sheol in Hebrew or Hades in Greek—because those who are there are deprived of the vision of God. Such is the case for all the dead, whether evil or righteous, while they await the redeemer: which does not mean that their lot is identical, as Jesus shows through the parable of the poor man Lazarus who was received into “Abraham’s bosom”: “It is precisely these holy souls, who awaited their Savior in Abraham’s bosom, whom Christ the Lord delivered when he descended into hell.” Jesus did not descend into hell to deliver the damned, nor to destroy the hell of damnation, but to free the just who had gone before him.
634 “The gospel was preached even to the dead.” The descent into hell brings the Gospel message of salvation to complete fulfillment. This is the last phase of Jesus’ messianic mission, a phase which is condensed in time but vast in its real significance: the spread of Christ’s redemptive work to all men of all times and all places, for all who are saved have been made sharers in the redemption.
635 Christ went down into the depths of death so that “the dead will hear the voice of the Son of God, and those who hear will live.” Jesus, “the Author of life,” by dying destroyed “him who has the power of death, that is, the devil, and [delivered] all those who through fear of death were subject to lifelong bondage.” Henceforth the risen Christ holds “the keys of Death and Hades,” so that “at the name of Jesus every knee should bow, in heaven and on earth and under the earth.”
“Our Lord’s expression is not to be understood of the earthly corporeal paradise, but of a spiritual one, in which all are said to be who enjoy the Divine glory. Accordingly, the thief descended locally into hell with Christ, because it was said to him: “This day thou shalt be with Me in paradise”; still as to reward he was in paradise, because he enjoyed Christ’s Godhead just as the other saints did.” - St Thomas Aquinas, Summa Theologica, 52
Source:
newadvent.org/summa/4052.htm