Before Christ died, those that died in sin, could not get into heaven. God created a thing called paradise—Abraham’s bosom—it wasn’t heaven, but that was where people went who had faith in God but were outside of Christ. When Christ took the sins of mankind on him, he made a way for us and them to go to heaven.
When he died he emptied paradise and brought them to heaven with him. When Christ died as a sinless man, no one had ever died being sinless. No one had ever come back from death. Death couldn’t keep him down because there was no sin in him.
“And when I saw Him, I fell at His feet as dead. But He laid His right hand on me, saying to me, ‘Do not be afraid; I am the First and the Last. I am He who lives, and was dead, and behold, I am alive forevermore. Amen. And I have the keys of Hades and of Death.’” (Revelations 1:17-18) Jesus marched over to the devil and took the keys to death and hell. Satan no longer controls the eternal destination of mankind. Christ redeemed us from eternal death.
To be clear, there are two kinds of death. Physical death, when the human conscience is separated from the body, and spiritual death, when our conscience is separated from God. The whole world was disconnected from God—spiritually dead—before Christ reconciled it in his body. “…He might reconcile them both to God in one body through the cross, thereby putting to death the enmity.” (Ephesians 2:16) Jesus rescued us from the uncertainty of physical death, but even more from the being the living dead. Now we are connected to God. His victory makes it very plain: the devil is defeated and Satan only reigns by deception, not authority.