On the cross, Christ told the repentant malefactor that that very day he would be with Him in paradise. But how could Christ make such a promise if the day of final judgment had not yet arrived? Paradise refers not to a final state but rather to the spirit-world experience of those who choose to follow Christ. Paradise is where these spirits wait for the resurrection and the day of judgment. The repentant malefactor had chosen to follow Christ, and so would be with Christ in “paradise”–the spirit world–after his death.
The apostle Peter taught that between Christ’s death and resurrection He visited and preached to the spirits in prison. “Prison” refers to the spirit-world experience of those who did not follow Christ in this life. (Note that the typical connotation of “prison” does not apply here. Many honest, sincere, wonderful people will find themselves in spirit “prison” simply because they never had the opportunity to learn of Christ’s true gospel or because they were sincerely dedicated to other belief systems.) Through the missionary work Christ initiated, as described in the verses below, many in the spirit world have chosen to accept Christ and now await baptism and other essential ordinances. Mormons perform these ordinances vicariously for their ancestors in LDS temples.
1 Peter 3:18-20: