Prior to the resurrection of Christ, the wicked and the righteous were completely separated from each other, there is a great gulf fixed: so that they which would pass from hence to you cannot; neither can they pass to us, that would come from thence (Lu 16:26). When the Lord set up the missionary program among the righteous, this great gulf was bridged so that the gospel could be preached to the spirits in prison, But behold, from among the righteous he organized his forces and appointed messengers, clothed with power and authority, and commissioned them to go forth and carry the light of the gospel to them that were in darkness, even to all the spirits of men; and thus was the gospel preached to the dead…Thus was the gospel preached to those who had died in their sins, without a knowledge of the truth, or in transgression, having rejected the prophets (DC 138:30-32). The reader is referred to Luke 16:19-31 and DC 138 which are great resources for understanding the spirit world.
James E. Talmage
“…many other great truths not known before, have been declared to the people, and one of the greatest is that to hell there is an exit as well as an entrance. Hell is no place to which a vindictive judge sends prisoners to suffer and to be punished principally for his glory: But it is a place prepared for the teaching, the disciplining of those who failed to learn here upon the earth what they should have learned…No man will be kept in hell longer than is necessary to bring him to a fitness for something better. When he reaches that stage the prison doors will open and there will be rejoicing among the hosts who welcome him into a better state.” (Conference Report, Apr. 1930, p. 97 as taken from Latter-day Commentary on the Book of Mormon compiled by K. Douglas Bassett, p. 342-3)