1 Peter 3:18-20 "For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God, being put to death in the flesh, but quickened by the Spirit: By which also he went and preached unto the spirits in prison; Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water."
Who are these "spirits in prison"?
Hebrews 1:7 "And of the angels He says: "Who makes His angels SPIRITS And His ministers a flame of fire."
Hebrews 1:13-14 says: "But to which of the angels has He ever said, Sit on my right hand, until I make your enemies your footstool? Are they not all ministering SPIRITS, sent forth to minister for those who will inherit salvation?"
Angelic beings are "spirits", so then could it be speaking of fallen angels in prison?
Several passages speak of a rebellion and fall during the "days of Noah".
Genesis 6:1-8 "Now it came to pass, when men began to multiply on the face of the earth, and daughters were born to them, that the sons of God saw the daughters of men, that they were beautiful; and they took wives for themselves of all whom they chose. And the LORD said, "My Spirit shall not strive with man forever, for he is indeed flesh; yet his days shall be one hundred and twenty years." There were giants on the earth in those days, and also afterward, when the sons of God came in to the daughters of men and they bore children to them. Those were the mighty men who were of old, men of renown. Then the LORD saw that the wickedness of man was great in the earth, and that every intent of the thoughts of his heart was only evil continually. And the LORD was sorry that He had made man on the earth, and He was grieved in His heart. So the LORD said, "I will destroy man whom I have created from the face of the earth, both man and beast, creeping thing and birds of the air, for I am sorry that I have made them." But Noah found grace in the eyes of the LORD."
The term "sons of God" is only used when speaking of angelic beings:
Job 1:6 "Now there was a day when the sons of God came to present themselves before the LORD, and Satan also came among them."
Job 2:1 "Again there was a day when the sons of God came to present themselves before the LORD, and Satan came also among them to present himself before the LORD."
The sons of God that presented themselves before the Lord were clearly angels.
Job 38:7 "To what were its foundations fastened? Or who laid its cornerstone, When the morning stars sang together, and all the sons of God shouted for joy?"
During the creation of the earth, no humans existed, only the sons of God, the angels, were present and shouted for joy.
Angelic beings, sons of God - not sons of men - saw the daughter of men and desired them and furthermore they had children with them! The result of this union were the giants, a race of beings that God had not created but were instead a product of an ungodly union between angels and men. This happened during "the days of Noah".
II Peter 2:4-5, 9 "For if God did not spare the angels who sinned, but cast them down to hell and delivered them into chains of darkness, to be reserved for judgment; and did not spare the ancient world, but saved Noah, one of eight people, a preacher of righteousness, bringing in the flood on the world of the ungodly;….. the Lord knows how to deliver the godly out of temptations and to reserve the unjust under punishment for the day of judgment,"
Jude 6-7 "And the angels who did not keep their proper domain, but left their own habitation, He has reserved in everlasting chains under darkness for the judgment of the great day; as Sodom and Gomorrah, and the cities around them in a similar manner to these, having given themselves over to fornication and gone after strange flesh, are set forth as an example, suffering the vengeance of eternal fire."
During the times of Noah, angels "left their own habitation" and went after "strange flesh", after the daughters of men. They are now "reserved in everlasting chains under darkness waiting for the judgment day."
Jesus went and preached, or made a proclamation to these angelic beings. Unfortunately scripture does not say what he proclaimed.
The Church of God Proclaimed