Happened (4819) (sumbaino) means to happen, with implication of occurring in connection w other events. The perfect tense treats what is certain to befall as already accomplished and as their final and permanent state!

True (227) (alethes see study on related word aletheia) means conforming to reality, unconcealed, manifest or in accordance with fact. Peter is stamping this proverb as in accord with reality.

Proverb (3942) (paroimia from pará = by, beside + oímos = a way, a highway) is literally something "by the way", a byword which is a short saying illustrating a general principle. It describes a pithy maxim giving expression to some observed event whose content has allegorical import. It also describes a brief communication containing truths designed for initiates and as such can be a veiled saying in which especially lofty ideas are concealed (used especially this way by John)

The fact that "proverb" is singular indicates that the message conveyed by the two examples is essentially the same, namely both point out examples of repulsive actions and to the character that these actions reveal. Simply put dogs will act like dogs thus showing that they were dogs all along and the same for pigs. The false actions of the teachers reflect who they really have been all along - false teachers. Furthermore this proverb was well known among the rabbis and the actions described was well known among the pagans.

In Peter's day dogs were not pampered pets like they are today! The Jews in fact spoke of the hated Gentiles as “dogs” because a dog was nothing but a filthy scavenger who ran in packs and lived on garbage!  Peter chooses a word which is the epitome of disrespect and revulsion to describe these false teachers who have known the truth but have turned away from it.

Michael Green

The dog which has got rid of the corruption inside it through vomiting it up cannot leave well enough alone; it goes sniffing around the vomit again. The pig that has got rid of the corruption outside by means of a scrubbing cannot resist rolling in the mud.

Returns (1994)(epistrepho) return to a point or area where one has been before, in this case to their own regurgitated gastric contents.

Vomit (1829) (exerama) means that which is thrown out (vomited). The picture here is not just of a dog "sniffing" the vomitus, but of actually lapping up what had been disgorged. Both dogs and pigs were considered vile by the Jews. The action of the dog (and the pig) reveals its true nature. The false teachers appeared to be renewed persons, having made professions of faith, but their false teaching and persistent practice revealed that they were still lost apostates.

Matthew Henry describes them as those who

have licked up their own vomit again, returning to the same errors and impieties that they had once cast off and seemed to detest and loathe, and wallowing in that filthiness from which they appeared once to be really cleansed.

J R Lumby...

Altogether become abominable: — To describe in all its horror the abysmal depth to which these false teachers have sunk, the apostle makes use of two proverbs, one of which he adapts from the Old Testament (Proverbs 26:11), while the other is one which would impress the Jewish mind with a feeling of utter abomination. The dogs of the East are the pariahs of the animal world, while everything pertaining to swine was detestable in the eyes of the Israelite. But all the loathing which attached to these outcasts of the brute creation did not suffice to portray the defilement of these teachers of lies and their apostate lives. It needed those other grosser features — the return to the disgorged meal; the greed for filth, where a temporary cleansing serves, as it were, to give a relish for fresh wallowing — these traits were needed ere the full vileness of those sinners could be expressed. (J. R. Lumby, D. D.) (The Biblical illustrator)

Vance Havner

The Bible compares us to different animals and some of the comparisons are not very complimentary. It says, "Don't be like a mule" (Ps. 32:9); a mule is usually backward about going forward! Jesus says that His sheep know Him and that sheep follow the shepherd; a sheep is not at home in a mudhole and a Christian is not satisfied to live in sin. There is an animal that feels at home in a mudhole; the Bible classifies the false teachers who return to their native habitat of sin with sows wallowing in the mire (2 Peter 2:22). We need to brush up on our Bible zoology. (2 Peter 2:22 Comment)

Thomas Reade

"A pig that is washed goes back to her wallowing in the mud." 2 Peter 2:22

The pig, though washed, was a pig still. The outward washing could not change the inward propensity of the animal. Is it not so with multitudes of baptized people? Outward reformation must never be confounded with inward regeneration!

"And when people escape from the wicked  ways of the world by learning about our Lord  and Savior Jesus Christ and then get tangled  up with sin and become its slave again, they  are worse off than before." 2 Peter 2:20

The hearts of such people were never renewed, their natures were never changed. This they proved by their return to the propensities of their natural hearts.

AND A SOW AFTER WASHING...TO WALLOWING IN THE MIRE: kai: us lousamene (AMPFSN) eis kulismon borborou:

Washing (3068) (luo) means to bathe oneself and specifically refers to washing the whole body and not part of it. The middle voice indicates that the washing was not forcibly applied against the pig's will but that the pig the washing himself. It pictures however an outer cleansing not an internal cleansing. The clear implication is that these false teachers had been fully exposed to the genuine gospel yet internally remained "full of dead men's bones". By the way don't press the proverb too far. Peter is not alluding to washing as a picture of baptism. The picture implies an outward cleansing form "defiling conduct". But the outward cleansing was readily undone by the pig's innate natural urge to find "relief" by returning to the mire. A pig can stay clean only a short time and then must head for the nearest mud hole. We do not condemn a pig for acting like a pig because it has a pig’s nature. If we saw a sheep heading for the mire, we would be concerned!

Peter’s point is that mere religious profession or even outward change does not change a person’s heart. These “professors but not possessors” seemed to some (especially the gullible) to have experienced salvation, but in due time they drifted back to the life that was a true reflection of their inner nature. Certainly the dog feels better after emptying his stomach, but it is still a dog. “Having an experience” did not change the dog's nature. In fact it only served as further proof of his “cannine nature,” because he came back and just like a dog lapped up his own vomit. What a disgusting picture of these vile false teachers!

The principle brought out by Peter calls for us all to apply this truth to our lives and reflect on our choices. Our choices are seen to be consistent with what we are. A good tree bears good fruit, a brackish spring pours forth brackish water, and the pig returns to wallowing in the mire.

What do my choices today and this past week, this past year, etc loudly proclaim about who I really am?

For heaven's sake, we need to be honest with ourselves!

The dog and pig in this context picture temporary external change resulting from conformity to a false profession of faith much like a chameleon blends with its surroundings whatever they might be. True faith is fruitful faith. False "faith" is shown by absence of good fruit (see Peter's earlier comments 2Pe 1:8, 9-note, 2Pe 1:10-note). Ignorance in the spiritual realm is not bliss but leads to fleshly indulgence. Unsaved people lack spiritual intelligence (Hos 4:6), and this causes them to give themselves to all kinds of fleshly and worldly indulgences (Acts 17:30; Ep 4:17 18, 19-note). Since we were born with a fallen nature it is natural for us to live sinful lives. Nature determines appetites and actions. A dog and a pig behave differently because they have different natures.

Jesus also used the designations "dogs" and "swine" in speaking of those opposed to God and his Word

Do not give what is holy to dogs, and do not throw your pearls before swine, lest they trample them under their feet, and turn and tear you to pieces. (Mt 7:6-note)

The principle of the animal illustrations is that like these animals, these false teachers were never what they seemed to be on the outside. They had never experienced a changed heart and been made new creatures in Christ (2Cor 5:17-note). To the contrary, they were still old creatures in Adam! (cp 1Co 15:22) And so it should not surprise us that return to those things that reflect their true nature. These charlatans are like dirty pigs which can be washed (cp Jesus' comparison of Pharisees to white washed tombs Mt 23:27) on the outside but on the inside are still dead in their trespasses and sins (Eph 2:1-note) and filled with hostility toward God and His will (Col 1:21-note)  Their hearts had never been washed clean by the blood of the Lamb and thus they were unable to behave in any way but "unclean". The irony is that the punishment for these fakers is that they incur a greater bondage to sin than before they masqueraded as teachers of the liberating truth of the Gospel! It is because of the very fact that false professors of a (pseudo) "new birth" return to their "pre-Christ" way which makes the doctrine of the perseverance of the saints so important. In other words those who persevere to the end of their life and never renounce their faith, prove by this perseverance that they are truly new creatures in Christ. This doctrine does not say that a person merits heaven by their personal perseverance in the faith but that they the fact that they do persevere proves there possess a power outside themselves which enables them to keep on keeping on.

The importance of perseverance in the faith helps us appreciate Peter’s earlier warning

Therefore, brethren, be all the more diligent to make certain about His calling and choosing you; for as long as you practice these things, you will never stumble (2Pe 1:10-note).

The writer of Hebrews gives a strong warning to after being exposed to the truth, turn away from the truth...

For if we go on sinning willfully after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, 27 but a terrifying expectation of judgment and the fury of a fire which will consume the adversaries. 28 Anyone who has set aside the Law of Moses dies without mercy on the testimony of two or three witnesses. 29 How much severer punishment do you think he will deserve who has trampled under foot the Son of God, and has regarded as unclean the blood of the covenant by which he was sanctified, and has insulted the Spirit of grace (Note: The writer is implying that these had hard the truth, possibly even that they had professed the truth)? 30 For we know Him who said, “Vengeance is Mine, I will repay.” And again, “The Lord will judge His people.” 31 It is a terrifying thing to fall into the hands of the living God. (Heb 10:26-31)

Ray Stedman

A review of the whole chapter shows that yielding to the money-mad, sex-obsessed, materialistic and anti-authoritarian drives of modern society are indications that an individual's heart is not in touch with the lordship of Christ, but has succumbed to the delusions of the devil instead. Pride in knowledge is the point of attack. (Commentary of 2 Peter Chapter Two)

Ron Ritchie sums up the fate of these fakes...

What will their payment be? Three things, Peter says. First, "For if, after they have escaped the defilements of the world by the knowledge of the Lord and Savior Jesus Christ, they are again entangled in them and are overcome, the last state has become worse for them than the first." Secondly, he says, they are like dogs which "return to their own vomit." They can't escape the shackles of their old life. Having gotten rid of their internal corruption, they find themselves returning to it again. Thirdly, he says, like pigs after washing, they "return to wallowing in the mire." They return again to the external corruption which they once had. What graphic terms Peter uses! Washing a pig does not change its heart; he still remains a pig. In the same way, a false prophet will always remain that, no matter how flattering his words, no matter how well he dresses. (What Are The Wages Of The False Teachers)

William MacDonald reminds us that

This passage should not be used to teach that true believers may fall from grace and be lost. These people never were true believers. They never received a new nature. They demonstrated by their last state that their nature was still unclean and evil. The lesson is, of course, that reformation alone is not only insufficient, but is positively dangerous, because it can lull a person into a false security. Man can receive a new nature only by being born again. He is born again through repentance toward God and faith in our Lord Jesus Christ (Ed: See comment on repentance below). (MacDonald, W & Farstad, A. Believer's Bible Commentary: Thomas Nelson or Logos)

Comment: See passages that emphasize the importance of repentance, a doctrine which a number of modern pulpits have "jettisoned"! Jonah 3:8,10 2Ki 17:13 2Chr 30:6 Pr 1:23 Isa 22:12 Jer 25:5 Ezekiel 14:6 18:30, 31, 32 33:11 Da 4:27 Hos 14:2 Joel 2:12 Mal 3:7 Ro 2:4 Rev 2:21 Mt 3:2 4:17 11:20 12:41 Mk 1:4,15 6:12 Lk 13:3, 5 15:7,10 16:30 24:47 Acts 2:38 3:19 5:31 8:22 26:20 17:30 20:21 2Co 7:9, 10 12:21 2Ti2:25 2Pe 3:9

Warren Wiersbe summarizes the characteristics of these false teachers:

We may detect them by their exaltation of themselves instead of Christ; their counterfeit talk and “great swelling words”; their emphasis on making money; their great claims that they can change people; and their hidden lives of lust and sin. For the time being we cannot stop them except by teaching the Word sincerely, but one day God will expose them and judge them...

It is a startling fact that there are many people in our churches who have never truly been born again, but who are convinced that they are saved and going to heaven!

They have had “an experience,” and perhaps look better (like the sow) and feel better (like the dog), but they have not been made better as “partakers of the divine nature. Perhaps Peter recalled Judas, one of the Twelve, who was a tool of the devil and was never born again. Up to the very end, the other disciples did not know the truth about Judas and thought he was a spiritual man! (Wiersbe, W: Bible Exposition Commentary - New Testament. 1989. Victor or Logos or Wordsearch) (bolding added)

Spurgeon ("Man's Thoughts and God's Thoughts")

As for peace in the hour of death, he who is not pardoned while living is not likely to be pardoned when dying. Nine out of ten, perhaps nine hundred and ninety-nine out of every thousand of professed death-bed salvations are a delusion.

We have good facts to prove that. A certain physician collected notes of several hundred cases of people who professed conversion when they thought they were dying. These people did not die as they had expected to, but continued to live. In the case of all but one they lived just as they had lived before, though when they were thought to be dying they appeared as if they were truly converted. Do not look forward to a death-bed salvation, it is a mere snare of Satan.

Alan Carr...

A person’s true nature will always come out! Just as a dog may be taught obedience and tricks, he will always be a dog and will have a dog’s nature and will eat his own vomit. A pig can be bathed, bowed and buffed to a high gloss, but it will always be a pig and will head straight for the wallow when turned loose. (Ill. Men can look good in church, but live in sin in the world. You will always do after you own nature!) This is why Christians can be assured of their security in Jesus. A new nature wants to avoid sin and please God. We are not reformed, we have been transformed – 2Co 5:17; 2 Pe1:4. (Sermons and Outlines)

Kenneth Gangel has some sound advice for all saints in these last, deceptive, difficult days

Believers today do well to heed Peter’s warning against false teachers, to learn how to discern truth for themselves, and to teach it to others. The false teachers will themselves meet destruction and others will be destroyed by them. But Christians can wage spiritual warfare more effectively if they know their spiritual enemies, the techniques that heretics use, and the end result of their deception." (bolding added) is, in itself, not similar) and we do not think and speak falsely but rightly when we describe the relationship as one of similarity. (Walvoord, J. F., Zuck, R. B., et al: The Bible Knowledge Commentary. 1985. Victor or Logos or Wordsearch)

J Vernon McGee summarizes this section in his unique pithy style