1 Peter 3:18-20 (ESV), 18 “For Christ also suffered once for sins, the righteous for the unrighteous, that he might bring us to God, being put to death in the flesh but made alive in the spirit, 19 in which he went and proclaimed to the spirits in prison, 20 because they formerly did not obey, when God's patience waited in the days of Noah, while the ark was being prepared,…”

Who were the spirits in prison?

The context tells us the spirits were the disobedient angels who took on fleshly bodies, mated with human women, and produced hybrid children (part angel/part human), the Nephilim. Genesis 6:2, 4 (ESV), 2 “the sons of God (the disobedient angels) saw that the daughters of man were attractive. And they took as their wives any they chose. … 4 The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of man and they bore children to them. These were the mighty men who were of old, the men of renown.”

When the flood came, all humans (except Noah’s family of eight people) drowned as did all the Nephilim. However, the fallen angels simply rid themselves of fleshly bodies. They did not drown. They returned to their natural state as spirit beings. God chose to punish Satan and these demons by confining them with chains of darkness in the earth’s atmosphere. Jude 1:6 (CSB), “and the angels who did not keep their own position but abandoned their proper dwelling, he has kept in eternal chains in deep darkness for the judgment on the great day.”

How did Jesus preach to the fallen angels?

Jesus preached to the spirits both before and after His resurrection. During the 3  1/2 years of His earthly ministry, Jesus’s holiness, love, and compassion for degenerate men must have astonished these angels. Then when He voluntarily died on the cross, that sermon clearly showed His supreme love for God and man.

After Jesus’s resurrection, He spent 40 days about earth before He ascended to heaven. The time Jesus spent reassuring and comforting His disciples hardly filled 40 days. Thus, it seems reasonable that He went to the spirit realm, to tartaroo (2 Peter 2:4), to preach to the repentant, imprisoned angels. He may have given them instruction and hope as to how they might change to  be re-instated to their holy status during the judgment day. We don’t know. Yet, Isaiah wrote, “For thus says the High and Lofty One Who inhabits eternity, whose name is Holy: ‘I dwell in the high and holy place, with him who has a contrite and humble spirit, to revive the spirit of the humble, and to revive the heart of the contrite ones.’” Isaiah 57:15 (KJV)