The word “spirits” could refer to people deceased and glorified as in Hebrews 12:23, to demons as in Matthew 8:16, Matthew 10:1, Acts 5:16 (indwelling people), to forces of darkness as in Ephesians 2:2, 1 Corinthians 12:10, 1 John 4:1, or to (obedient) angels as in Hebrews 1:7, Hebrews 1:14. The fact that these spirits are imprisoned would limit the possibilities in the present context to those hostile to the Lord, namely, demons and/or forces of darkness. It is to be noted that in his second letter, Peter mentions that God, after the angels’ rebellion in the beginning (they are now known as demons), “committed them to chains of gloomy darkness to be kept until the judgment” (1 Peter 2:4; Jude 1:6).
19 in which he went and proclaimed to the spirits in prison,