Another definition from my above quoted source:
HERESY (hehr’ eh ssee) An opinion or doctrine not in line with the accepted teaching of a church; the opposite of orthodoxy. Our English word is derived from a Greek word which has the basic idea of choice. In ancient classical Greek it was used predominantly to refer to the philosophical school to which one chose to belong. Later, it came to be associated with the teaching of philosophical schools.
The word had a similar usage in Jewish writings. Josephus, a Jewish historian of the first century from whom we learn much of what we know about the Judaism of New Testament times, used the word to refer to the various Jewish parties (or schools of thought) such as the Pharisees, Sadducees, and Essenes. Jewish rabbis employed the term in a bad sense applying it to groups who had separated from the main stream of Jewish teaching.
The word has several usages in the New Testament, but never has the technical sense of “heresy” as we understand it today. It may be classified as follows:
1. Most frequently, especially in Acts, it has the same meaning as Josephus. In Acts 5:17, 15:5; and 26:5, where it refers to the Pharisees and Sadducees, it simply means party or sect.
2. In Acts 24:14 and 28:22 it is used in a slightly derogatory sense, referring to Christians as they were viewed to be separatists or sectarians by the Jews. This usage conforms to that of the rabbis.
3. Paul used the term to refer to groups which threatened the harmonious relations of the church. In 1 Corinthians 11:19, where he was writing about the disgraceful way in which the Corinthians were observing the Lord’s Supper, the word has to do with the outward manifestations of the factions he mentioned in verse 18. In Galatians 5:20, it is one of the works of the flesh and is in a grouping including strife, seditions, and envyings. It apparently has to do with people who choose to place their own desires above the fellowship of the church. Titus 3:10 speaks of a man who is a heretic. Since the context of the verse has to do with quarreling and dissension, the idea in this passage seems to be that of a fractious person.
4. In 2 Peter 2:1 it comes closest to our meaning of the term. It clearly refers to false prophets who have denied the true teaching about Christ. Since the remainder of 2 Peter 2 refers to the immoral living of the false prophets, the word also refers to their decadent living. The reference to the heretic in Titus 3:10 may belong to this category since the verse mentions disputes about genealogies, a doctrinal matter.
It is clear that in the New Testament, the concept of heresy had more to do with fellowship within the church than with doctrinal teachings. While the writers of the New Testament were certainly concerned about false teachings, they apparently were just as disturbed by improper attitudes.
In the writings of Ignatius, a leader of the church in the early second century, the word takes on the technical meaning of a heresy. Most frequently in the writings of the early church fathers, the heresy about which they were concerned was Gnosticism, a teaching which denied that Jesus was fully human. See Christology; Error; Gnosticism.
W. T. Edwards, Jr.