Okay, Jesus came into the world so that whoever believes in him will have eternal life. So, per the christian bible, believers go to heaven after they die. Where did they go BEFORE Jesus died on the cross?

This is the teaching on that

Hades has two compartments, the abode of the lost called in the Bible "torment," and the other side, the place of the saints of God called Paradise, and also referred to as "Abraham's Bosom."

When Jesus gave up His Spirit to death, He went into the Paradise side of Hades and "took captivity captive." This means He took all the souls of the saints who had died to that point to be with God the Father in Heaven. The Paradise side of Hades is now empty. The torment side today holds all souls of people who rejected Christ's salvation, and who have died to this point in human history. People who are saved by believing in Christ today, and who die, now go directly to God, the Father, in Heaven.

"For as Jonah was three days and three nights in the belly of the great fish, so will the Son of Man be three days and three nights in the heart of the earth" (Mt. 12:40).

"Because You will not leave my soul in Hades, or allow Your Holy One to see decay" (Acts 2:27).

"Wherefore he saith, When he ascended up on high, he led captivity captive, and gave gifts unto men. Now that he ascended, what is it but that he also descended first into the lower parts of the earth?" (Eph. 4:8-9).

:) G'Day : What I understand about this subject is that we sleep in the dust of the Earth until the resurrection, our flesh sees corruption but at the resurrection it is "reconstituted", as it was in the garden at the beginning, I will rise again as "I myself" this corruption must put on incorruption, the righteous at the first resurrection unto eternal life and the condemned at the second resurrection unto eternal destruction, this has been the case since the day of righteous Abel.
God's judgment is not as mens justice, i.e.. Revenge, but is of righteousness, and has nothing to do with good and evil, if you look at Genesis you will see that good and evil comes from the same tree, but righteousness comes from God alone imputed to those whom He has chosen, the tree of Life.
Blessings Miykle