I think I answered this before elsewhere, but I’ll repeat it here.
Ask yourself, what caused the Fall of Man, and what happened after it? Adam and Eve sinned and refused to repent, and were cast out of Paradise with the snake. The snake represented their sin, or that which separated them from God - hence, mankind was cast into a fallen world with their sin. That sinful nature separated them from God, even unto death, for before the Resurrection all the dead went into the what the Psalmist called “the pit,” and which Jewish tradition called
Sheol.
Now, remember that mankind was made in God’s image, and when mankind sinned that image of God was tarnished (Christ referred to this in His parable of the lost coin, Luke 15:8-10). When they died, that image remained tarnished, and they were dead in their sins.
This is why the Incarnation happened. At the Incarnation, the Eternal Word took on our tarnished image from the Virgin Mary but maintained His deity with the Holy Spirit - it’s known as the hypostatic union, which states that Christ had a Divine and Human nature as well as a Divine and Human will. He was both fully God and fully Man.
Christ lived a sinless life, and at the Crucifixion presented Himself as the final sacrifice for the people before God - only this time, it was different. There would be no Temple sacrifices, and this time it would be for
all who believe, not merely the Jews. When Christ went down to
Sheol as many did before Him, something took place that had never happened before - the Gates of Hades could not comprehend His divine nature, and they broke down. All who had died before and who believed in their Lord ascended to Paradise with Christ.
The most important part, however, took place at the Resurrection. Christ rose again in a spiritually glorified body, and conquered death for all who believe in Him. In the Orthodox Church we have a chant at
Pascha that perfectly describes this moment:
*Christos anesti ek nekron, thanaton thanatos…
Christ is risen from the dead, by death conquering death…*
When Christ ascended into Paradise, He ascended with that glorified flesh, and in this manner reunited mankind with God once again. No longer are we disconnected with God because of what we are, for now we are able, through Christ, to rise again and be connected with God, for through the Incarnation God united us to Him. As Saint Athanasius said, God became man so that man may become God - not by nature but by imitation.
That is it in a nutshell.