The passage reads:
18 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, 19 in which he went and proclaimed to the spirits in prison, 20 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. (1 Peter 3:18-20, ESV)This is a notoriously difficult passage. There are two major interpretations:
First, there are those who believe the passage teaches that after his crucifixion, and before his resurrection—while his body lay dead in the grave—Christ himself (in his own disembodied spirit) preached in the underworld to the spirits of those who perished in the flood.
But who were these "spirits"? Some say they were the righteous dead of the Old Testament era, and that their “prison” was the abode of the dead prior to the coming of Christ (the righteous dead being denied heaven until Christ’s ascension). The message he preached is believed to be that he had made atonement for their sin and they were now granted access to heaven.
Others say that the "spirits" to whom Christ preached were the wicked dead of the Old Testament era, their prison was hell, and the message he preached was the eternal condemnation of the wicked.
Second, there are those who believe that Christ, through the Holy Spirit, and in the person of Noah, preached to people in Noah’s day who were eventually destroyed by the flood. It is pointed out that Peter calls Noah “a herald of righteousness” (2 Pet. 2:5), and that prophets such as Noah are said to have spoken by “the Spirit of Christ in them” (1 Pet. 1:11). Further, Christ is said to have carried on this ministry of preaching by the Holy Spirit: “having been…made alive in the spirit (or, by the Spirit); in which (or, by whom) he also went and made proclamation,” etc. In this view, the “spirits” to whom Christ preached (through the Holy Spirit and in the person of Noah) were the people who were alive in Noah’s day. They were not disembodied spirits when they were preached to, but were so when Peter was writing. They were not spirits in “prison” when they were being preached to, but living men roaming the earth; at the time of Peter’s writing, however, they were in “prison,” that is, in hell; the NASB supplies the important word “now”: “the spirits now in prison”. The message which was preached to them as living men in Noah’s day was a message of repentance (cf. 2 Pet. 2:5).
I think this is a better interpretation of the passage is better than the first.