The verse that mentions the spirits in prison, 1 Peter 3:19, is a difficult one, but if we look carefully at the passage, and compare it with other references in Scripture, we can deduce its meaning. The verse in question says "he (Jesus) went and proclaimed to the spirits in prison." The first thing we need to know is where Jesus was for the three days between His death and resurrection. That issue requires its own article, which can be found here.
What spirits in prison did Jesus preach to between His death and resurrection? Are the spirits in prison related to the Nephilim?
Next, we need to know the identity of the spirits. The word translated "spirits" in this passage is from the Greek pneumasin, and it means "air, breath, wind." This word is used to refer to true spiritual beings, including angels (Hebrews 1:14) and demons (Mark 1:23) and Jesus' spirit (Matthew 27:50) or the Holy Spirit (John 14:17). The passage is referring to spiritual entities, not the spirits of human beings, who are never referred to as "spirits" even after death. The usage of the word tells us that the spirits in prison are not, and never were, human beings.
If the spirits in prison are not human or holy angels, there is only one option: they are demons. However, we know that demonic activity continues on the earth, based on many examples from the New Testament. Who are the spirits in prison? Why are they imprisoned? If not all demons are in prison, what happened to cause these specific spirits to go to prison, and why did Jesus go and preach to them?
There is a clue in the book of Jude, verse 6: "And the angels who did not stay within their own position of authority, but left their proper dwelling, he has kept in eternal chains under gloomy darkness until the judgment of the great day." This is speaking about a group of demons who did something grievous, and earned eternal imprisonment. This passage is very similar to the one in 1 Peter 3:19-20, which says "he went and proclaimed to the spirits in prison, because they formerly did not obey…"
The rest of v. 20 reads as follows: "…when God's patience waited in the days of Noah, while the ark was being prepared…" This is another major clue as to the identity of the spirits in prison. If their sin was committed during the days of Noah, specifically while "God's patience waited" during the building of the ark, it is likely that the spirits in prison are the same spirits who "saw that the daughters of man were attractive" and "took as their wives any they chose" and produced children who were giants and very mighty. These were called Nephilim, and apparently contributed greatly to the wickedness on the earth in those days (Genesis 6:1-4). This interpretation is further supported by the fact that the spirits in prison were said to have abandoned their proper dwelling place to do whatever they did to earn their imprisonment. It is very likely that the "spirits in prison" are the same spirits who left the spiritual realms to mate with human women and produce unholy children, the Nephilim.
Nephilim - Who or what were they?
What are the principalities and powers the Bible talks about?
Where was Jesus for the three days between His death and resurrection?
Did Jesus and Satan fight over the keys to the kingdom? What are the keys to the kingdom?
Why should I believe in the resurrection of Jesus Christ?
Truth about Angels