Commentaries:
The apostle is speaking about the efficacy of Christ's suffering and death in making possible a relationship between God and human beings. His conclusion, beginning in I Peter 4:1-2, is that, since Christ suffered so much to bring this about, Christians should respond by "ceas[ing] from sin" and living "for the will of God."
This means, of course, that in doing so, we no longer live as we used to, like the "Gentiles," like the world (verse 3). Seeing this, our friends who are still in the world wonder why our lives have changed so drastically, and they are likely to malign us for it (verse 4). But we need not worry because God, the just Judge, will bring them into account for their abuses of us (verse 5). In verse 6, he winds up his discussion by providing a general example to give us hope in this regard. He explains that the gospel had been preached in the past to people who are now dead, and even though their contemporaries may have judged them worthy to suffer persecution and death, God, conversely, has judged them worthy of eternal life. He implies that God would do the same for us.
Richard T. Ritenbaugh
Jesus and 'the Spirits in Prison'