20 If indeed they have escaped the corruption of the world through the knowledge of our Lord and Savior Jesus Christ, only to be entangled and overcome by it again, their final condition is worse than it was at first. 21 It would have been better for them not to have known the way of righteousness than to have known it and then to turn away from the holy commandment passed on to them. 22 Of them the proverbs are true: “A dog returns to its vomit,” and, “A sow that is washed goes back to her wallowing in the mud.”2 Peter 2:20-22 BSB
Starting from the last verse, a dog and a pig illustrate an unregenerate person. The vomit and mud symbolize a sinful lifestyle. On the one hand, you have the body ejecting something harmful from within, and the other is washed clean from the outside. Both illustrations, the dog and the pig: reflects a person who at one point did 180 degrees through the conviction of the Holy Spirit. However, the person’s change of lifestyle from sin to holiness was only momentary.
In the second half of verse 20, the person seems to get caught up again with their old sin habits. This time though, the sin that the person left at first when he returns to it again becomes a stronghold in his life worst than the first time and much more harder to overcome.
In verse 21, it gives a stern warning to those who know the truth of the gospel and yet are being tempted to abandon the Lord for the pleasures of this life. The outcome will be that in hell, it will be much worse for an individual like that than for the one who never knew the whole truth of Jesus Christ and died in that state.
According to Romans 1:19-20, no one is without excuse when it comes to knowing the existence of God. For nature itself points to Him, which reveals God’s power and divine nature. If the person responds to that revelation, Jesus will then reveal Himself by sending a missionary. But if there are none, He will send an angel with the gospel (Revelation 14:6-7).
A genuine Christian will never leave Jesus (John 10:27-30), but a false convert has a high potential of denying Jesus or worshiping a different Jesus.