2 Peter 2:4

(4) For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment; 
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Regarding “the angels who sinned”, the Bible asserts that “God did not spare” them, meaning that He has not pardoned their sins, just delayed their punishment. The verse goes on to say that, in the meantime, He has “cast them down to hell [tartaroo] and delivered them into chains of darkness, to be reserved for judgment.”

E.W. Bullinger writes that their prison, Tartarus, “is not Sheol or Hades, . . . [but] denotes the bounds or verge of this material world” (A Critical Lexicon and Concordance to the English and Greek New Testament, “hell,” p. 370). Tartarus, then, is a holding place—this material world—where the demons are awaiting their final judgment. Their ultimate penalty is not “chains of darkness” or “everlasting chains under darkness” (Jude 6), but something far more permanent to be rendered in “the judgment of the great day.” This appointed time of judgment still awaits them (see Matthew 8:29).

Paul writes unambiguously that “the wages of sin is death” (Romans 6:23). God also says in Ezekiel 18:4, 20, “The soul who sins shall die.” Scripture does not stipulate that this applies only to humans (soul means “living being”—even God is a soul; see Leviticus 26:11, 30; Isaiah 1:14; Jeremiah 6:8; Zechariah 11:8; Matthew 12:18; Hebrews 10:38; etc.), nor does God’s Word ever say that sin can be paid for by a lengthy, even eternal, imprisonment (as many speculate will be the demons’ fate). According to these verses, all sin requires death for expiation, and since the Bible does not indicate that demons will repent of their sins and accept Jesus Christ’s death to pay for their transgressions, only their own deaths will cover their many terrible sins.

— Richard T. Ritenbaug