Isn’t 2 Pet. 2:20-21 proof that you can lose salvation? No.
look at the CONTEXT of 2 Pet 2. Some people say, "When I read 2 Pet.
2:20-21 in its full Biblical context, I don't see how it reads
differently than when it stands on its own." Are you blind? Look at the
verse numbers that follow below and the subject of those verses from 2
1. False prophets, false teachers, damnable heresies denying the Lord that bought them, swift destruction.
2. Pernicious ways, the way of truth shall be evil spoken of.
3. Covetousness, feigned words, judgment, damnation.
4. They are compared with the fallen angels.
5. They are compared to Noah's day, the world of the ungodly.
6. They are compared to Sodom & Gomorrah.
7. The filthy conversation of the wicked.
8. Their unlawful deeds
9. To reserve the unjust ... to be punished.
10. Walk after the flesh, despise government, presumptuous, self willed, speak evil of dignities.
12. As natural brute beasts, destroyed, speak evil, shall utterly perish in their own corruption.
13. The reward of unrighteousness, riot, spots, blemishes, own deceivings.
14. Eyes full of adultery cannot cease from sin, beguiling unstable souls, cursed children.
15. Forsaken the right way, gone astray.
17. Wells without water.
18. Great swelling words of vanity, wantonness.
19. The servants of corruption.
If you can't see that in the context, then one of two things is true. 1. You truly are blind. 2. You are only using the Bible to try to prove that you are right, and that your own opinion about salvation must be the Biblical opinion. In either case, you are sadly mistaken. The whole context in 2 Pet. 2 is about false teachers and preachers who know the way of righteousness and who know that Jesus Christ is the Lord and the Savior, but who have willfully rejected His personal salvation because of their lust, self will, presumption, covetousness, and deceit.
Some say that they can't believe that the devil or false teachers and preachers know the Lord. Well, the Bible says the devil does know him. But I don't expect you to believe the Bible if it doesn't back up your opinion. I fully expect that you will find a way to reject the verses that I'm about to show you so that you don't have to believe that they are true. But just look:
James 2:19, the devils BELIEVE that there is one God.
Mark 1:24, "I KNOW thee who thou art, THE Holy One of God." You know who said that? An unclean spirit said that.
Mark 1:34, Jesus "suffered not the devils to speak, because they KNEW him."
Mark 5:6-7, a man possessed with Legion devils WORSHIPPED Jesus saying, "...Jesus, thou son of the most high God."
Acts 16:16-18, a woman possessed with a spirit of divination said of Paul repeatedly, "These men are the servants of the most high God, which shew unto us the WAY OF SALVATION." Then he cast out that spirit.
Some say, "I don’t believe (that's a mouthful there) 'the knowledge of Jesus Christ' can mean a non Christian or even demonic experience." Well, there are five verses or references from the Bible showing beyond the shadow of a doubt that the devils believe in God, worship Him, know Him, know that He is THE God, and know that His is the way of salvation. Just because some refuse to believe that doesn't make them right and the Bible wrong. Instead it makes them a judge of God's words. Now, that's dangerous. The devil and Eve got together and judged God's words in Gen. 3 and she and the entire human race suffered death as a consequence of that boo-boo. Welcome to the crowd.
According to 2 Pet. 3:18, a man can grow "in the knowledge of our Lord and Saviour Jesus Christ" if he is saved. Notice that he said, "OUR Lord." 2 Pet 2 said, "THE Lord." But if he is lost, that knowledge is going to be used against him and his latter end is going to be worse than the beginning if he doesn't repent and get saved.
Hope this helps.