Peter is warning about false teachers that profess to be Christians but still keep a tight hold on their old sinful ways of life…showing that their knowledge of Christ and the Way of righteousness was only superficial. At this time, these false teachers were teaching & advocating that you could have your cake & eat it too…kind of like having one foot in heaven while still having one foot in the world. But in doing so they were flatly denying the Lordship of Christ and leading many astray. The Proverb Peter was quoting here is Proverbs 26:11, which concludes that fools don’t learn from their mistakes….and these false teachers obviously had not learned either that they couldn’t have it both ways.