The phrase “in prison” sends our thoughts to a place (e.g., hell). The Greek, though, does not require us to think locationally; it simply wants us to think of the circumstance of imprisonment (notice the absence of the definite article, “the prison”). In 2 Peter 2:9 the apostle uses parallel language when he says that God “keeps” the unrighteous under punishment (also Matthew 27:36), that is, imprisonment with no reference to location.
Thus, though held in prison,
these spirits may, under God’s providence (1 Samuel 16:14; 1 Kings 22:21–22; Job 1:12; Job 2:6), exert an influence on mankind (Ephesians 2:2; Ephesians 6:12; Revelation 13:1–18). That would include moving Israel’s leaders to crucify Christ (Acts 2:23) as well as move unbelievers to subject Christians to suffering on account of the faith. Through the work of Jesus Christ on the cross, however, they have been soundly defeated (Colossians 2:15; Revelation 12:7–8).
19 in which he went and proclaimed to the spirits in prison,