Are the imprisoned spirits in 1 Peter 3:19 human spirits or fallen angels?
Answer: They are human spirits.
II. Did [demons] worry about being imprisoned by Jesus? Was there a prison for demons [in] Peter's time?
Answer: Demons are already imprisoned. The prison is Hades.
I would suggest that all imprisoned spirits were at one time human, and I will attempt to demonstrate that in this response.
Now, perhaps our first consideration should be this:
How could any angel in a paradise of absolute perfection, holiness, grandeur, and majesty, one who shares in the magnificence and intellect of an infinite God, possibly ever fall?
If they can do so, what does this say about our future with God? Could we, who are inferior to angels, not easily fall from heaven thus repeating the catastrophe in the Garden? And, why would God ever allow angels to fall? They are His ministering spirits (Heb. 1:14), not a band of traitorous, diabolical insurrectionists as many would have us believe (cf. 2 Pet. 2:10, Jude vs. 8: "reviling angelic majesties").
Some will immediately protest: "We know that Satan is a fallen angel!" Really? Just how do we know that? Naturally, we may then be lectured over at least two "go-to" passages that everyone uses to support this idea:
Isaiah 14:12: “How you have fallen from heaven, O star of the morning (often mistranslated "Lucifer" by Jerome), son of the dawn!"
"There you have it! Lucifer." Everyone who believes in "fallen angels" seems to overlook the fact that the taunt in Isaiah was against the king of Babylon:
Isaiah 14:4: "[You] will take up this taunt against the king of Babylon"
Add to that, the fact that the "morning star" is merely a reference to the planet Venus, and it should become apparent that the verse is not describing "Lucifer" - the purported name of Satan which itself is in error. Note how Christ describes Himself:
Revelation 22:16: "I am the root and the descendant of David, the bright morning star."
Would Christ ever associate Himself with the Devil, especially through His very identity? Others will point to Ezekiel, chapter 28. What they also tend to overlook is that the taunt in that chapter is clearly to be spoken against the ruler of Tyre:
Ezekiel 28:2: "“Son of man, say to the leader of Tyre…"
It is a matter of fact that God often uses celestial imagery to describe His judgments on people and nations. There should be no mystery about this.
Here are some of the other common instances where we (understandably) draw conclusions about angels. Note the copious bracketed explanations:
Genesis 6:4: "The Nephilim [Nephilim were very large sons of Anak in Ex. 13:33] were on the earth in those days—and afterward as well—when the sons of God [godly lineage of Seth] had relations with the daughters of men [faithless female descendants of Cain]."
1 Peter 3:19-20: "[Christ was] made alive in the spirit; in which also He went and [preached through Noah to people whose] spirits [are now in prison as Peter writes his Letter], who once were disobedient, when the patience of God kept waiting in the days of Noah"
Yes, disobedient human beings refused the message of Christ preached through Noah, and were lost, their spirits now languishing in the prison of Hades as Peter writes. This should demonstrate that there are, at this very moment, spirits in prison. (Later, as I attempt an explanation of demons; this will be valuable because it illustrates how the inhabitants in Hades exist in time whereas Hell is eternal.)
1 Peter 4:6: "For the gospel has for this purpose been preached even to those who are [now] dead [but were long enough to receive the Gospel], that though they are [were] judged in the flesh as men [all of us are so], they may live in the spirit according to the will of God [because they were saved while still alive, and now departed]."
2 Peter 2:4-10: "For if God did not spare angels [the line of Seth polluted by daughters of Cain?] when they sinned, but cast them into [Tartarus] and committed them to pits of darkness, reserved for judgment [in Hades]; and did not spare the ancient world [later during Noah, also a descendant of Seth], but preserved Noah [the last faithful man alive from Seth's lineage], a preacher of righteousness, with seven others, when He brought a flood upon the world of the ungodly [people]; and if He condemned the [people of the] cities of Sodom and Gomorrah to destruction by reducing them to ashes, having made them an example to those [people] who would live ungodly lives thereafter; and if He rescued righteous Lot, oppressed by the sensual conduct of unprincipled [people]…"
Jude vs. 5-7: "Now I desire to remind you, though you know all things once for all, that the Lord, after saving a people out of the land of Egypt, subsequently destroyed those who did not believe. And angels [priests, messengers, Korah among the Israelites] who did not keep their own domain [in rebellion to Moses], but abandoned their proper abode [station as priests], He has kept in eternal bonds under darkness for the judgment of the great day, just as [the people of] Sodom and Gomorrah and the [people of the] cities around them, since they in the same way as these indulged in gross immorality and went after strange flesh [sexual perversions], are exhibited as an example in undergoing the punishment of eternal fire."
Job 4:18: "'He puts no trust even in His servants; And against His angels [messengers, priests, etc.] He charges error."
Are we to leap to the conclusion that all such passages are referring to "fallen angels?" Why can these not be further references to God's human messengers, those that make mistakes (there is an abundance of examples)? Do heavenly, angelic majesties make the same mistakes as human beings? It is possible. But that proves nothing relative to the concept of "fallen angels."
The term "angel" is very elastic, often (as the bracketed notation above suggests) simply meaning messenger, priest, prince, etc. There are other variations as well. I propose that too often, we tend to believe that every mention of angels is referring to God's angelic majesties. Some definitely are (cf. Job 1:6, 2:1). If we read a verse from Hebrews, it might at first appear that God gives no assistance to angels, which seems puzzling:
Hebrews 2:16: "For assuredly He does not give help to angels, but He gives help to the descendant of Abraham."
Someone might immediately claim victory: "Fallen angels! And they need help. Why? Because they 'fell'." Obviously, the assumption, one that seems valid, is that angels need help. But how do we know this? There is no doubt many other passages exist that one can find if they are determined to advance this concept. I, however, suggest they are making unwarranted assumptions without due consideration and evidence. It is easy to fall into the trap of believing something because you have repeatedly heard it many times before.
Finally, what about demons? What are they, exactly? Well, it turns out that we do have some information about them. Here is something from Luke's Gospel:
Luke 8:30-31: "And Jesus asked [the demon possessed man in the tombs], 'What is your name?' And he said, 'Legion'; for many demons had entered him. They were imploring Him not to command them to go away into the abyss."
Note the word "abyss" in verse 31. This is important because it appears we are cautioned against speculating over who may be saved, and who may be lost:
Romans 10:6b-7: "“DO NOT SAY IN YOUR HEART... ‘WHO WILL DESCEND INTO THE ABYSS?’" (cf. Deu. 30:12).
This is clearly not referring to angels, fallen or otherwise. It is referring to lost human beings that face the very real prospect of imprisonment in Hades. The more we contemplate the unlikelihood of "fallen angels," and instead focus on the identity of demons, the more it should become apparent that unclean spirits are "unclean" because they were never washed in the blood of he Lamb; they refused to accept God on His terms.
Instead, they rejected God's offer and now (1 Pet. 3:19) exist as nothing more than demons, spirits of the lost. They are not "fallen angels" despite the insistance otherwise. We have become so conditioned to this erroneous concept that we tend to resort to its use out of habit.
I do not have the ability to relate much more on demons in this response. But imagine a vicious, dead giant from the days of Noah, a blood thirsty killer that would never hesitate to destroy you, me, and everyone around him. What might he do if given one unique opportunity?
Luke 11:24: "When the unclean spirit goes out of a man, it passes through waterless places [the rich man in Hades pleaded for water] seeking rest, and not finding any, it says, 'I will return to my house [human body] from which I came [possessed].'"
It seems that unlike angelic majesties that have no sexual component (Matt. 22:30, Mk. 12:25) and cannot die (Lk. 20:36), demons always yearn to be clothed with a human body. Is it not at least plausible that a demon would feel most comfortable in human flesh because it was once similarly clothed?
There is so much more that could be discussed in this matter; I've merely scratched the surface in this lengthy response.