We all know Christianity teaches that Jesus died on Good Friday and came back from the dead on Easter Sunday, but what did he do in the meantime? If your answer is, "Nothing! He was dead," remember that belief in the afterlife is an important part of Christianity. The Apostles' Creed actually includes a line saying that Jesus descended into Hell, and traditions developed around this idea. The Gospel of Nicodemus, an apocryphal Latin work often conflated with the Acts of Pilate, includes a section about Jesus' visit to Hades. While some English translations use "Hell" as the equivalent of "Hades," this isn't entirely accurate. Hades, the Greek world of the dead, contained a few different sections. One of them, Tartarus, was basically the equivalent of the modern Hell, being a place of eternal torment. It was, however, reserved for the worst of the worst. Most people actually ended up in the dull, boring Plain of Asphodel, which doesn't seem to have been all that much different from the Jewish concept of Sheol. The spirits of the dead whom we see in the Gospel of Nicodemus don't seem to be tortured so much as just waiting around. John the Baptist, a relatively new arrival in Hades, preaches the importance of repentance to the dead, basically insisting that it's their only chance to get out. Adam and Seth are both present at this sermon, and Seth tells a story about how, when Adam lay dying, he journeyed to the gates of Paradise to ask for a cure. An angel showed up and told the younger patriarch that he'd have to wait until 5500 years after the creation of the world, at which point God's son would fix everything.
I find the figure of 5500 years given here to be interesting. We don't know exactly when Jesus died, but it would have been around the Jewish year 3790. So there was apparently some tradition at the time of the writing of the Gospel that the world was older than Jewish scholars thought it was, although only by a negligible amount when compared to the actual age of the planet. Even today, there isn't a complete consensus among Young Earth Creationists as to the world's age, although a lot of them still hold to the seventeenth century calculations of Dr. John Lightfoot and Bishop James Ussher, who regard the year of creation as 4004 BC. Some argue, however, that the world is closer to 10,000 years old. Given what we now know, the fact that the argument even still exists is ridiculous, but science wasn't quite as developed in medieval times.
After Adam and Seth say their piece, the story cuts to a conversation between Satan and Hades, both of whom are fretting about how Jesus plans to rescue some souls from the underworld. Jesus then shows up, breaks the gates of Hades, and binds Satan in chains. He then takes the patriarchs, prophets, martyrs, and forefathers with him to Heaven, where they meet Enoch, Elijah, and the man who was crucified next to Jesus.
While the Gospel of Nicodemus obviously never obtained canonical status, the idea that Jesus came to Hades to save the patriarchs remained popular. Dante mentions it in his Inferno, although he limits it to eight individuals the Messiah saved from Limbo: Adam, Abel, Noah, Moses, Abraham, David, Jacob, and Rachel. The descriptions in Nicodemus make it sound like he took a lot more people than that, and Dante doesn't even mention Seth. I assume this tradition was in answer to the question of what happened to the people who were good, faithful followers of God BEFORE Jesus made his atoning sacrifice. I believe some other Christians think that Jesus' sacrifice actually took place outside of time, with the crucifixion by the Romans being merely an earthly representation of the eternal truth. While I don't know for sure, I'm guessing that this means people could have been saved prior to Jesus' earthly life. We know that Christianity developed at a time when mystery cults were popular among the Romans, and the leaders of these religions taught that the myths were less important than the mystical, universal messages behind them. One of the main rivals to Christianity was the worship of Mithras, and it seems that the main thing he did was kill a bull, which is no mean feat but hardly something you'd think could save the world.
I get the impression that, even early on in the development of Christianity, people were skeptical of the idea that a guy (even if he WAS the Son of God) being executed would atone for all the sins of the world. Hence, they came up with explanations that gave a deeper and more mystical significance to this earthly event. It doesn't appear that Mel Gibson wanted any of this, though, as it seems like the main message of the Passion of the Christ was "you should believe in Jesus because he went through a lot of crap." While I'm certainly not minimizing crucifixion, probably the most inhumane means of execution ever devised, Jesus wasn't the only one who had to go through it, and other people have suffered severe pain for much longer periods of time. So it strikes me that Mel's theology might well be less sophisticated than that of people 2000 years ago, but is anyone really all that surprised by this?