Who are the “spirits in prison” to whom Christ preached? (1Pet. 3:18-20).

This passage links up with 2Pet. 2:4 and Jude 6. Note the following:

1. Human beings are never referred to in the Bible as “spirits” without qualification (e.g. “the spirits of just men made perfect”). 
2. Angels and demons are referred to as “spirits” without qualification. 
3. Those to whom Christ preached “disobeyed … while the ark was being built.” 
4. The Bible speaks of angels who sinned in the days of Noah saying that they were imprisoned in Tartarus (2Peter 2:4,5) – not the usual words for hell (Hades, or Gehenna). 
5. Jude gives similar testimony (Jude 6). 
6. Perhaps these rebellious angels were the “sons of God” in Genesis 6 (Genesis 6:1-4 c.f. Job 38:4-7). 
7. The word used here for preaching is not the Greek word that specifically means, “to preach the gospel”. It simply means, “to proclaim.” The content of the proclamation is not stated. 
8. The flow of the passage moves from Christ’s death, to his resurrection, to his proclamation to the spirits. 
9. The subject of his proclamation was, most likely, his resurrection victory over sin and Satan’s forces.
The meaning is thus made clear: While Noah was building the Ark evil angels appeared in the likeness of men and took to themselves human women. These evil angels were arrested and placed in suitable confinement to await their final judgment. To these spirits in prison, immediately after his resurrection, Christ went and proclaimed his victory!

Such an interpretation fits well with the immediate context that links the resurrection of Christ with his exaltation and ascension into heaven to be at God’s right hand. It says nothing about the state of human beings in death.