The "spirits" in prison are NOT demons, but human beings back in Noah's time. Noah was a type of Christ, peaching in the spirit of Christ, to the old world. Compare 1 Peter 3:19 with 2 Peter 2:5. Also, people in the bondage of sin are also referred to as being in chains and darkness, as seen in Psalm 107:9-16. Here is how the Spirit of Prophecy explains 1 Peter 3:19 : "God here makes Noah a great blessing to the world, and herein makes him an eminent type of the Messiah, though not the Messiah himself, as his parents expected, Genesis 5:29. 1. God made him a preacher to the men of that generation. As a watchman, he received the word from God's mouth, that he might give them warning, Ezekiel 3:17. Thus, while the long-suffering of God waited, by his Spirit in Noah, he preached to the old world, who, when Peter wrote, were spirits in prison (1 Peter 3:18-3:20), and herein he was a type of Christ, who, in a land and age wherein all flesh had corrupted their way, went about preaching repentance and warning men of a deluge of wrath coming." MHBCC 7.23