Verse 18 is communicating the thought that Jesus died to the physical world, but was made
alive to the spiritual world as the first resurrected man. This resurrection signified a victory that
had never before been attained. A physical man entered the spiritual dimension fully restored
to life. This indicated the battle with sin, death, evil and the dark kingdom of Satan had been
secured. Paul writes of this in First Corinthians:

“When the perishable has been clothed with the imperishable, and the mortal with immortality, then the saying that is written will come true: ‘Death has been swallowed up in victory.’
                         ‘Where, O death, is your victory?
                        Where, O death, is your sting?’
The sting of death is sin, and the power of sin is the law. But thanks be to God! He gives us the victory through our Lord Jesus Christ.
- First Corinthians 15:54-57

Peter continues in verse 19 to tell us that after  “being made alive” (or, resurrected) Jesus went
and “made proclamation” of this victory to the rebellious spirits (the descendants of the nephilim
of Genesis 6) who had been “imprisoned” (placed in everlasting chains until judgment in Tartarus which is found in the underworld of 2 Peter 2:4 and Jude 6, 7) for their disobedient “in the
days of Noah while the ark was being built.”  Jesus’ proclamation may have sounded similar
to his words to John on the isle of Patmos:

“I am the First and the Last. I am the Living One; I was dead, and now look, I am alive for ever and ever! And I hold the keys of death and Hades.”
– Revelation 1:17-18

"Who builds a church to God, and not to Fame,
Will never mark the marble with his Name."
- Alexander Pope