2 Pet. 2:1 says, “But false prophets also arose among the people, just as there will also be false teachers among you, who will secretly introduce destructive heresies, even denying the Master who bought them, bringing swift destruction upon themselves.” If Jesus only paid for the Christians, then why does it say that the false teachers denied the Lord who bought them?

This Question can seem difficult one for Calvinists who teach that Jesus only died for the elect.  But, we need to interpret the Scriptures in harmony with other scriptures. For example, John 19:30  is what Jesus said "it is finished" always been crucified. That term "It is finished" is the Greek 'tetelestai'  and that word  has been found at the bottom of ancient papyri  legal document signifying that the debt had been paid in full. Also, Col. 2:14 says, "[Jesus] having canceled out the certificate of debt consisting of decrees against us and which was hostile to us; and He has taken it out of the way, having nailed it to the cross," (NASB).  We clearly see the legal payment has been made and that the debt has been canceled. If Jesus bore the sin of everyone who ever lived, and the certificate of debt has been canceled, then all should go to heaven; otherwise, their debt hasn't been cancelled.  But since not all people go to heaven (Matt. 25:46; Mark 3:29), we know he could not have paid for everyone's debt. Furthermore, the Bible says that people do not lose their salvation.  1 John 2:19 says, "They went out from us, but they were not really of us; for if they had been of us, they would have remained with us; but they went out, in order that it might be shown that they all are not of us."  So, what do we do with 2 Pet. 2:1?  Is it saying that people lose their salvation or that Jesus paid for all people's sins? Neither.

Peter addressed his first epistle to the Jews who were aliens scattered abroad (1 Pet. 1:1), just as James was addressing "the twelve tribes who are dispersed abroad," (James 1:1). In both epistles Peter referred to the Old Testament many times:  1 Pet. 1:22-25; 2:6-9; 2:24; 3:8-12; 2 Pet. 2:5-8; 2:21; 2 Pet. 3:4-6.
Therefore, it seems natural to conclude that he is referring to the Old Testament when he speaks about people being bought.  Consider Deut. 32:6 says, “Do you thus repay the Lord, O foolish and unwise people? Is not He your Father who has bought you? He has made you and established you.” Moses was addressing the rebellious Israelites who were turning away from God, “Is not He your Father who has bought you?” Peter says in 2 Pet. 2:1 that "there arose among the people" probably referring to the Old Testament times and then says "just as there will also be false teachers among you."  Peter switches from past tense to future tense referring to false teachers of past times, and false teachers to come.  It makes sense to see that Peter is referring to the Jew's being freed from their bondage in Egypt.  But Peter it is not talking about being born again, being redeemed, or having their sins paid for.  He's talking about the false teachers of old and the new who were delivered from the bondage of Egypt, who should have listened to God' prophet (Moses) but rebelled against the work of God provided for them.