Alarming as II Peter 2:1-3 is, Peter does not define heresy, but he does tell what one heresy is and will be. He also does not tell us here what the source of heresy is either.
Heresy is the translation of the Greek hairesis—meaning literally "choice" or "selection"—which has an interesting secular as well as biblical history. Until its biblical use, it had no evil connotation. Even in the Bible, it is mostly used to refer to a party or a philosophy with which a person had chosen to identify or ally himself. Thus, hairesis is frequently translated "sect." In Acts, Luke applies it to the Sadducees (Acts 5:17) and the Pharisees (Acts 15:5; 26:5). Outsiders also used hairesis in Acts 24:5, 14 and Acts 28:22 to identify the Christian church.
However, when Paul and Peter's writings began circulating, hairesis meant a destructive element within the church that creates division through consciously formed opinions and ideas in disagreement with the orthodox teachings of the apostles. Paul condemns it in Galatians 5:20 as one of "the works of the flesh." Sometimes it is translated "factions" or "party spirit," but regardless of its translation, Paul says that people who practice such things will not inherit the Kingdom of God (verse 21)!
In the ordinary course of secular life, heresy was of little consequence; one person's opinion or choice about most things in life is just as good as another's. A person can be given any number of alternatives, any one of which he may be perfectly free to believe. However, in Christianity we are dealing with revelation, with God-given truth, with absolutes. When God's truth comes to men, we either have to accept or reject it. Thus, a heretic is a man who believes what he wishes to believe instead of accepting the truth of God that he ought to believe.