1. Men are never spoken of in Scripture as "spirits". Man has a spirit, but he is not "a spirit", for a spirit hath not flesh and bones".  In this life man has "flesh and blood", a "natural" (or psychical) body.  At death this spirit "returns to God Who gave it" (Ps. 31:5.  Eccles. 12:7.  Luke 23:46.  Acts 7:59).  In resurrection "God giveth it a body as it hath pleased Him" (1Cor. 15:38).  This is no longer a "natural" (or psychical) body, but a "spiritual body" (1Cor. 15:44).
   
   
2. Angels are "spirits", and are so called (Heb. 1:7, 14).
   
   
3. In 2Pet. 2:4 we read of "the angels that sinned"; and in 1Pet. 3:19, 20 of spirits "which sometime were disobedient ... in the days of Noah".  In 2Pet. 2:4 we are further told that the fallen angels are reserved unto judgment, and delivered into chains (i.e. bondage or "prison").  Cp. Jude 6.
   
   
4. The cause of their fall and the nature of their sin are particularly set forth by the Holy Spirit in Jude 6, 7.
   
   
   1. They "left their own habitation".
      
      
   2. This "habitation" is called (in Greek) oiketerion, which occurs again only in 2Cor. 5:2, where it is called our "house" (i.e. body) with which we earnestly long to be "clothed upon"; referring to the "change" which shall take place in resurrection. This is the spiritual resurrection body of 1Cor. 15:44.
      
      
   3. This spiritual body (or oiketerion) is what the angels "left" (whatever that may mean, and this we do not know). The word rendered "left", here, is peculiar. It is apoleipo = to leave behind, as in 2Tim.  4:13, 20, where Paul uses it of "the cloke" and the "parchments" which he left behind at Troas, and of  Trophimus whom he left behind at Miletum.  Occ. Heb. 4:6, 9; 10:26.  Jude 6.
      
      
   4. They "kept not their first estate (arche)" in which they were placed when they were created.
      
      
   5. The nature of their sin is clearly stated. The sin of "Sodom and Gomorrha" is declared to be "in like manner" to that of the angels; and what that sin was is described as "giving themselves over to  fornication, and going after strange flesh" (Jude 6, 7).  The word "strange" here denotes other, i.e. different (Gr. heteros = different in kind.  See Ap. 124. 2)  What this could be, and how it could be, we are not told.  We are not asked to understand it, but to believe it.  (see further in App.  23 and 25).
   
   
   
5. In Gen. 6:1 ,2, 4 we have the historical record, which is referred to in the Epistles of Peter and Jude. There these "angels" are called "the sons of God".  This expression in the Old Testament is used always of "angels", because they were not "begotten", but created, as Adam was created, and he is so called in Luke 3:38 (cp. Gen. 5:1).  It is used of angels eight times:  Gen. 6:2, (*1) 4.  Job 1:6; 2:1; 38:7.  Ps. 29:1 (R.V.m.); and Dan. 3:25.  In this last passage there is no article, and it does not mean "the Son of God", but "a son of God", i.e. an angel who was sent into the furnace (Dan. 3:28), as one was into the den of lions (Dan. 6:22).  In one passage (Hos. 1:10) the English expression is used of men, but there the Hebrew is different, and it refers only to what men should be "called", not to what they were.
   
   
6. Returning to 1Pet. 3:19, the expression "the spirits in prison" cannot be understood apart form the whole context.  The passage commences with the word "For" (v. 17), and is introduced as the reason why "it is better, if the will of God should (so) will, to suffer for well-doing, than for evil-doing. FOR (v. 18) Christ also suffered for sins once (Gr. hapax) - a Just One for unjust ones - in order that He might bring us to God, having been put to death indeed as to [His] flesh, but made alive as to [His] spirit."  This can refer only to His spiritual resurrection body (1Cor. 15:45).  In death His body was put in the grave (or sepulcher, i.e. Hades), Acts 2:31; but His spirit was "commended to God".  Not until His spirit was reunited to the body in resurrection could He go elsewhere.  And then He went not to "Gehenna", or back to Hades but to Tartarus (2Pet. 2:4.  See Ap. 131. III), where "the angels who sinned" had been "delivered into chains".  To these He proclaimed His victory.
   
   
7. The word rendered "preached" is not the usual word euangelizo (Ap. 121. 4), but the emphatic word kerusso (Ap. 121. 1); which means to proclaim as a herald.  Even so Christ heralded His victory over death, and the proclamation of this reached to the utmost bounds of creation.
   
   It was "better" THEREFORE to suffer for well doing than for evil doing.  He had suffered for well doing. He suffered, but He had a glorious triumph.  "Therefore" (runs the exhortation), "if ye suffer for righteousness' sake, happy are ye" (vs. 14), and it concludes "Forasmuch then as Christ suffered on our behalf as to the flesh, arm yourselves likewise with the same mind; for He that hath suffered in the flesh hath done with sin; no longer to live [our] remaining time according to men's lusts, but for God's will... For to this end, to those also who are now dead, were the glad tidings announced, that though (Gr. men) they might be judged according [to the will of] (*2) men, in [the] flesh, yet (Gr. de) they might live [again] according to [the will of] God, in [the] spirit" : i.e. in resurrection (1Pet. 4:1, 2, 6).
   
   The above is suggested as the interpretation of the expression "the in-prison spirits", in the light of the whole of the nearer and remoter contexts.

(*2) For the supply of this ellipsis see Rom 8:27, 28, and cp. 1Pet 4:19.