Peter uses knowledge in the same way as he did in 2 Peter 1:2, a reference to personal knowledge of God, life lived in relationship with him. This is the knowledge that brings conversion and salvation (2 Peter 1:2–3, 2 Peter 1:8).1 From this we can discern that the heretics have known Christ in the sense that they have become part of his people. They identify themselves as Christian believers and are seen as Christians by the rest of the church. Because they had/have every appearance of being Christian, Paul focuses in his description of them on what people can see, and not on what they actually are.2 He talks about them as if they were genuine believers, although they are not.
20 For if, after they have escaped the defilements of the world through the knowledge of our Lord and Savior Jesus Christ, they are again entangled in them and overcome, the last state has become worse for them than the first.