This difficult passage has been widely interpreted in three ways:
One interpretation says that these were the spirits of people who had died, and that Christ preached the good news to them after his resurrection. However, the New Testament does not support the idea that people can hear the good news or respond to it after death. Because of this, it is not likely that spirits in this passage refer to the spirits of dead people.
A second interpretation understands 3:19–20 as describing Christ preaching through Noah to the people who lived in Noah’s time. These people are now spirits in prison. This interpretation would explain the reference to people in the days of Noah in 3:20. However, it does not fit well with the main discussion about Christ’s death and resurrection in 3:18.
In the third interpretation, the spirits in prison are evil spiritual beings. Jewish tradition, based on Genesis 6:1–4, taught that many angels rebelled during the time of Noah (see also Jude 1:6; 1 Enoch 6–10). In this view, Peter is saying that Christ announced his victory over these evil spiritual powers after his resurrection (compare 1 Peter 3:22).