In 1 Peter 3:18, we read, "For Christ also suffered once for sins, the righteous for the unrighteous, that he might bring us to God, being put to death in the flesh but made alive in the spirit." This is the gospel. We can all say, "Hallelujah" after hearing that passage. Then we come across 1 Peter 3:19-20, which says, "in which he went and proclaimed to the spirits in prison, because they formerly did not obey, when God's patience waited in the days of Noah, while the ark was being prepared, in which a few, that is, eight persons, were brought safely through water."
These are verses that will make our head scratch. Why is this in there? What does it mean that Christ proclaimed the spirits in prison? Scholars have debated about this passage for some time. According the ESV Study Bible, there are three interpretations, which the first two most likely fall in line with Christian doctrine, which the third one seems a little far fetch:
(1) The first interpretation understands “spirits” (Gk. pneumasin, plural) as referring to the unsaved (human spirits) of Noah’s day. Christ, “in the spirit” (1 Pet. 3:18), proclaimed the gospel “in the days of Noah” (v. 20) through Noah. The unbelievers who heard Christ’s preaching “did not obey … in the days of Noah” (v. 20) and are now suffering judgment (they are “spirits in prison,” v. 19). Several reasons support this view: (a) Peter calls Noah a “herald of righteousness” (2 Pet. 2:5), where “herald” represents Greek kēryx, “preacher,” which corresponds to the noun kēryssō, “proclaim,” in 1 Pet. 3:19. (b) Peter says the “Spirit of Christ” was speaking through the OT prophets (1:11); thus Christ could have been speaking through Noah as an OT prophet. (c) The context indicates that Christ was preaching through Noah, who was in a persecuted minority, and God saved Noah, which is similar to the situation in Peter’s time: Christ is now preaching the gospel through Peter and his readers (v. 15) to a persecuted minority, and God will save them.
(2) In the second interpretation, the spirits are the fallen angels who were cast into hell to await the final judgment. Reasons supporting this view include: (a) Some interpreters say that the “sons of God” in Gen. 6:2–4 are angels (see note on Gen. 6:1–2) who sinned by cohabiting with human women “when God’s patience waited in the days of Noah” (1 Pet. 3:20). (b) Almost without exception in the NT, “spirits” (plural) refers to supernatural beings rather than people (e.g., Matt. 8:16; 10:1; Mark 1:27; 5:13; 6:7; Luke 4:36; 6:18; 7:21; 8:2; 10:20; 11:26; Acts 5:16; 8:7; 19:12, 13; 1 Tim. 4:1; 1 John 4:1; Rev. 16:13–14; cf. Heb. 1:7). (c) The word “prison” is not used elsewhere in Scripture as a place of punishment after death for human beings, while it is used for Satan (Rev. 20:7) and other fallen angels (2 Pet. 2:4; Jude 6). In this case the message that Christ proclaimed is almost certainly one of triumph, after having been “put to death in the flesh but made alive in the spirit” (1 Pet. 3:18).
(3) In a third view, some have advocated the idea that Christ offered a second chance of salvation to those in hell. This interpretation, however, is in direct contradiction with other Scripture (cf. Luke 16:26; Heb. 9:27) and with the rest of 1 Peter and therefore must be rejected on biblical and theological grounds, leaving either of the first two views as the most likely interpretation.
Still scratching your head? You are not alone. Even Martin Luther could not understand this passage. We know the verses following talk about Jesus being resurrected and ascending on high at the right hand of God. Passages like this may stump us but it should never cause us to lose faith. We may get it wrong, but one day, we will not need to worry what this passage means because we will see Jesus as He is.