Watch out for false teachers!
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Their teachings are destructive
However, there were also false prophets among the people, just as, indeed, there will be false teachers among you, who will introduce* destructive heresies, even denying the Owner who bought them (bringing on themselves swift destruction). And many will follow their licentious ways, because of which the way of the Truth will be defamed. And in covetousness they will exploit you with deceptive words; since way back the judgment for such people is not slack, and their destruction will not sleep.
Their doom is sure
Because if God did not spare angels when they sinned, but delivered them into chains of darkness, confining them in Tartarus§ to be reserved for judgment, and did not spare the ancient world but preserved Noah, with seven others, a preacher of righteousness, when He brought a flood on the world of the ungodly, and condemned the cities of Sodom and Gomorrah to destruction, reducing them to ashes (providing a warning to those who are intent on being ungodly),* while He rescued upright Lot, who was distressed by the licentious lifestyle of the wicked (for that upright man, dwelling among them, tormented an upright soul from day to day by seeing and hearing lawless deeds) then the Lord knows how to deliver the godly out of testings and to reserve the unjust under punishment for the day of judgment, 10 and especially those who go after the flesh in the lust for defilement,§ and despise authority.
Their depravity is ‘unreal’
They are audacious, self-willed; they are not afraid to revile majestic beings,* 11 whereas angels, though being greater in might and power, do not bring a reviling accusation against them before the Lord. 12 But these, as unreasoning animals, creatures of instinct made to be caught and destroyed, spouting off in matters of which they are ignorant, will be destroyed by their own depravity,§ 13 receiving the wages of unrighteousness. Counting it a pleasure to carouse in the daytime, they are stains and blemishes, reveling in their own deceptions* while they feast with you, 14 having eyes full of adultery and that cannot cease from sin, enticing unstable souls; having a heart trained in greediness they are children of a curse; 15 having forsaken the right way§ they have gone astray, having followed the way of Balaam of Bosor, who loved the wages of unrighteousness; 16 but he got a rebuke for his wrongdoing—a mute donkey speaking with a human voice restrained the insanity of the prophet.* 17 These are waterless springs, clouds driven by a storm, for whom the blackest of the darkness has been reserved forever.§
Their victims lose out
18 Because speaking pompous words of emptiness they entice through fleshly lusts, through licentiousness, those who have actually escaped from those who are living in error, 19 promising them freedom while they themselves are slaves of corruption; for by whom a person is overcome, by him also he is brought into bondage. 20 For if, after they have escaped the defilements of the world through the real knowledge of the Lord and Savior Jesus Christ, they are again entangled in them and overcome, the latter circumstances have become worse for them than the first ones. 21 For it would have been better*

for them not to have really known the way of the righteousness, than having really known it to turn back from the holy commandment delivered to them.
22 But it has happened to them according to the true proverb: a dog returns to its own vomit and a bathed sow to wallowing in mud.
* 2:1 The verb here does not normally imply secrecy; purveyors of false ideas usually come on as sincere, above board, confident, even bold. 2:1 Sometimes we wish that destruction were a little ‘swifter’, but 3:9 below gives the answer. 2:2 The biblical worldview is defamed. § 2:4 Tartarus is not to be equated with Hades (the holding area for departed human spirits) nor with the Lake of Fire, since the Text here clearly says those angels are being reserved for judgment. The day when Satan and his angels will be cast into the Lake is still future (Revelation 20:10, Matthew 25:41). So Tartarus is a prison for certain fallen angels—for more about those angels see 1 Peter 3:19-20 and Jude 6-7. * 2:6 Unfortunately, “those who are intent on being ungodly” could care less about any such warnings, even if they have any awareness of them. 2:8 Compared to the people around him Lot was definitely “upright”, but evidently it was mostly a passive uprightness; he did not make any converts, and even lost his immediate family. 2:9 They are under punishment already before the Day of judgment. Compare Luke 16:19-31, which is not said to be a parable: in Hades (which is not the Lake of Fire), before the final judgment, the rich man was already being punished (when someone is put in prison he is already being punished, before his case is tried and sentence pronounced). In Hades there is a great gulf between the just and the unjust. Since we determine our destination by what we do in this life, and there is no second chance (the rich man knew he was condemned), the punishment, or the blessing, starts at physical death. § 2:10 People who are greedy in their moral depravity tend not to respect (or even acknowledge) authority, including that of angelic beings. * 2:10 Literally ‘glories’—based on the next verse, it appears that they are high ranking fallen angels. 2:11 Greater than unregenerate human beings (presumably not than those seated in Christ—Ephesians 1:20-21, 2:6). 2:11 Compare Jude 9. § 2:12 Depravity destroys the quality of life already, down here, even before death and judgment. * 2:13 “Reveling in their own deceptions”—I imagine all of us know people like this. 2:14 The eyes don't stop sinning. I am reminded of our Lord's words in Matthew 6:22-23. “The lamp of the body is the eye. So if your eye is sound your whole body will be full of light. But if your eye is evil your whole body will be full of darkness. So if the light that is in you is darkness, how great is that darkness!” Of course we have two eyes, but the Text has ‘eye’ in the singular. I take it that the reference is to the way we interpret what we see (which is our real ‘eye’)—two people, one pure and one vile, observing the same scene will give very different interpretations to it. ‘Evil’ here has the idea of malignant, aggressively evil. Someone with a malignant mind will give an evil interpretation to everything he sees, and in consequence his being will be filled with unrelenting darkness. Compare Titus 1:15. 2:14 Peter, a Jew, uses a Jewish idiom—they are characterized by curse. They are a walking curse, damaging everyone they touch. § 2:15 Their first decision was to forsake the right—once you turn out the light you condemn yourself to stumble around in the dark. * 2:16 Peter says that what Balaam did was ‘insane’, just like the mercenary ‘prophets’ of our day. 2:17 Such clouds often rush past without dropping any water. The false teachers make all sorts of promises, but being empty they cannot deliver; they cheat their hearers. 2:17 This darkness is associated with Satan's kingdom. Peter is saying that they will share Satan's destiny. The description given to these false teachers is unusually severe, Peter really does not like them! Since God hates those who have sold themselves to do evil (Psalm 5:5-6, Deuteronomy 7:10), so does Peter, and so must we. (Psalm 97:10 has a command: “You who love Jehovah, hate evil!” David, at least, obeyed it—Psalm 26:5, 31:6, 119:113, 139:21-22. How about us?) God's love includes hating evil, of necessity, because of the consequences of the evil. If God loves us He must be against whatever may harm us. § 2:17 Just over 2% of the Greek manuscripts, of objectively inferior quality, omit “forever” (as in NIV, NASB, LB, TEV, etc.). * 2:21 Oh my, batten down the hatches! If it would have been better for them not to have known, then of necessity they are now worse off than they were before they knew (as verse 20 makes clear). But before they knew they were lost, so what can be worse than that? To be lost without a chance. Before, they were lost but had the option to be saved. But once they know the Truth, if they throw it away there is no second chance. Hebrews 6:4-6 is plain enough (if you hear someone twist the Text into a pretzel in an effort to avoid the plain meaning, you may be sure that whatever position he is trying to defend is incorrect). “For it is impossible for those who were once for all [hapax] enlightened, and have tasted the heavenly gift, and have become sharers in the Holy Spirit, and have tasted the good Word of God and the powers of the coming age, and have fallen away—it is impossible to renew them again to repentance, thereby crucifying the Son of God again, just for themselves, holding Him up to contempt.” You cannot crucify again unless you have crucified at least once already. For someone to crucify Jesus again must mean that he has indeed taken advantage of the first crucifixion, he was indeed saved—if he was never really saved he never really appropriated the first crucifixion, so he cannot do it again. I have used ‘to really know’ to translate the heightened form επιγινωσκω (in contrast to γινωσκω, ‘to know’) and ‘real knowledge’ to translate the heightened form επιγνωσις (in contrast to γνωσις, ‘knowledge’). I consider that the Holy Spirit is an intelligent and honest user of human language, so His choice of the heightened forms was deliberate. Those who seek to arrive at the intended meaning of the Text need to respect that deliberate choice. When Peter says someone has a “real knowledge of the Lord and Savior Jesus Christ”, that someone must be saved. To elevate a doctrinal system above the Word of God is a form of idolatry.