If we read 2 Peter chapter 2, it is clear that the false prophets and teachers in view are doomed to eternal condemnation. But what about those deceived by them? Some read verses 20-22 and conclude that believers who follow false teaching lose their salvation or prove that they were never true believers to begin with. Both of those interpretations clash with the clear biblical teaching that faith in Jesus Christ is the sole condition for salvation, and that salvation cannot be lost. Observing the context helps us understand their fate.
Clearly, the false prophets and false teachers mentioned at the beginning of chapter 2 are unsaved and doomed to eternal destruction. The passage starts with a contrast between them and the "holy men of God" mentioned in the preceding passage ("But" in 2:1; cf. 1:21). The language describing their eternal doom is explicit and unequivocal (2:3-17; as also in the parallel description in Jude 4-16).
It appears there is a second group of people in this passage— those influenced by these false teachers to the point that they "follow their destructive ways" (2:2). These also appear to be unsaved, since they are in contrast to the saved readers, a third group addressed directly in verse 3 ("you"). The second group profess salvation which causes the Christian way to be blasphemed when they are seduced.
As mentioned, the third group is the readers Peter is warning. He certainly addresses his epistle to believers only: They share the same "precious faith" and "righteousness of our God and Savior Jesus Christ" (1:1); God has given the readers all they need to live a godly life (1:3); They "may be partakers of the divine nature" (Peter could be speaking about their present position or perhaps a future privilege earned by their righteous behavior—either way it assumes their salvation; 1:4); They have "escaped the corruption that is in the world" (1:4). After that affirming introduction, Peter exhorts the readers to add to their initial faith godly virtues (1:5-7) so that they will not be barren (or useless, from argos), unfruitful, shortsighted, or blind lest they forget (or neglect to appreciate) that they were cleansed from their sins (1:8-9).
These Christian readers need to be warned about the false teachers and their seduction of those not saved (2:2). After describing the doom of the false teachers and their unsaved followers, Peter directs his attention to this third group in verse 18. The change in address is clear. After indicting the false teachers in every verse from verse 10 through 17, the indictments stop abruptly. In verse 18 and 19 Peter describes how the false teachers seduce gullible believers. The victims are described as "the ones who have actually escaped from those who live in error." The present participle used here could be translated "are escaping" or "are barely escaping," but it is clear from the other two uses of this verb in 2 Peter (1:4; 2:20) that the escape is actual. The potential victims are contrasted with those unbelievers in verse 2 who do not escape the false teachers.
When Peter speaks of the fate of the believing readers who could be influenced by the false teachers, he speaks in terms of possibility, not certainty. The "they" in verse 20 refers to those potential victims of verses 18-19. Nevertheless, it is a real warning about real consequences. As believers, they have "escaped the pollution of the world through the knowledge of The Lord and Savior Jesus Christ" (v. 20). The fact that they can be "again entangled" in the world's pollution means that they had once been, but had escaped (v. 20). Peter's statement in verse 21 makes it clear that they had "known the way of righteousness," a reference to an intimate knowledge (epiginōskō) of the Christian way. Their fate, described by the two contemporary proverbs cited in verse 22, requires that the dog had once left its vomit, and the pig was once washed.
So, we see a shift in Peter's address. He writes to this group of believers to warn them of the unsaved false teachers among them who will be destroyed and have led others to the same fate (vv. 1-17). Then in verse 18 he addresses the believers who are also coming under the influence of the false teachers. These believers face a terrible fate, but it is not specified as destruction or hell, only that their "latter end is worse for them than the beginning" (v. 20). Whatever struggles or trials they endured as new Christians will fade in comparison to the trouble that awaits them (perhaps temporally, or at the Judgment Seat of Christ, or both). Peter says it would have been better if they had not known "the way of righteousness" than to turn from "the holy commandment delivered to them" (v. 21). Peter is not saying that it would be better if they had never been saved. He is saying that it would be better if they had not known the teaching about the life of righteousness implying that because they do, they have a greater responsibility to follow it. The text indicates that this life of righteousness is defined by living according to "the holy commandment delivered to them." What is this holy commandment? Apparently it is not related to any command to believe or be saved, as that would be an unusual and unprecedented way of referring to salvation. It is likely the command to be holy (1 Pet. 1:15), a command to Christians.
It is naive to say that true Christians will not follow false doctrine. Like Peter, the other New Testament authors were not convinced of such a notion—see their epistles which contain many warnings to Christians about staying in the truth. Or, ask any pastor of tenure who has seen Christians come and go with the strangest of doctrines. It is especially sad to see and disconcerting to know that unless they repent of their error, there is a devastating fate that awaits them. It is best to avoid false teachers altogether and warn Christians about them. Not only is their doctrine false, but their motives are subversive to God's purposes.