even though it could simply mean that God, foreknowing their resurrection to life, regards them as the living, though they are literally dead.
The fact that some of those to whom Christ was preached have died is therefore no basis for judging the value of the gospel. God will judge rightly. The Christian dead may have indeed been judged by human standards in this life and may have been found wanting, whether by popular opinion or by official action. Nevertheless, judged by God’s standards, they are alive in the eternal realm of the Spirit. Because this verse is sometimes used to support the possibility of conversion after death, the reasons for rejecting this interpretation deserve further consideration. The referent of “the dead” (νεκροῖς, nekrois) in 4: 6 must be informed by the use of the same term in 4: 5, where it forms half of a merism that refers to all humanity in all ages, whether physically alive at the moment or physically dead. Therefore, the understanding, ancient though it may be, that 4: 6 refers to the spiritually dead is unlikely. Hilary of Arles (ca. AD 401– 449) expresses this understanding and the possible connection to 3: 19: “The gospel is preached to the Gentiles who are dead in sin, but this may also refer to the fact that when the Lord was buried in the tomb he went to preach to those who live in hell” (Bray 2000: 113). If even ancient commentary allowed that nekrois might refer to the physically dead, it raises the question of who these dead were and specifically if they were the same beings that Christ preached to in 3: 19. Those who understand 3: 19 to be a reference to a descent into hell, where Christ preached the gospel in a postmortem offer of salvation, have construed 4: 6 to be a broadening of that principle, even though the verses have few points of contact. S. Johnson (1960) argues for this interpretation based on a rather artificially constructed chiasm and overlooks the fact that the two verses do not occur within the same discourse unit. The immediate contexts of 3: 19 and 4: 6 should take priority in informing their respective interpretations. This is especially true since the two verses are only superficially similar. In 3: 19 Christ is the one who proclaims, but in 4: 6 the verb is passive and implies that Christ is the content of the preaching. This problem has sometimes been answered by broadening the postmortem preaching to extend to preaching done by the deceased apostles. Furthermore, the verbs are not the same in both verses, for the more general verb κηρύσσω (kēryssō, proclaim) stands in 3: 19, but εὐαγγελίζομαι (euangelizomai, preach good news) is a more specific reference to preaching the gospel in 4: 6. The weightiest reason the two verses are not directly related is that the audience in 3: 19 is “the spirits” (pneumata), not “the dead” (nekrois) as in 4: 6, and the two words are not synonymous. It was the assumption that Christ descended to Hades, as stated in the Apostles’ Creed, that gave rise to the theory of postmortem conversion in 4: 6 (see comments on 3: 18– 22). Goppelt (1993: 289) is one of the few interpreters who argues that the wording of 4: 6 “suggests that proclamation of the gospel is encountered by the dead when they are dead and that their death here, as in v. 5, is literal” (emphasis added). He reads 4: 6 in the context of 3: 19 as an eschatological event where the proclamation of Christ applies not only “to the most lost but to all the dead” (1993: 289). Therefore, in his judgment both 3: 19 and 4: 6 are mythological images that should be understood “as a kerygmatic confession, without trying to objectify it as an order of salvation for the dead or as a portrayal of a Hades proclamation.”
Jobes, Karen H.. 1 Peter (Baker Exegetical Commentary on the New Testament) (pp. 358-359). Baker Publishing Group.