Concise Bible Dictionary:
The word αἴρεσις is from “to choose.” The same Greek word is translated “sect” and is applied to the sects among the Jews, as the Sadducees and the Pharisees (Acts 5:17; Acts 15:5; Acts 26:5). It was employed by the Jews respecting Christianity (Acts 24:5,14; Acts 28:22). Heresies and sects were developed early in the church, they were the result of the working of the will of man in some form. The root of the Greek word being “to choose” shows that a heresy is something peculiar. The doctrine held and enforced may be true in itself but may be exaggerated or put out of its connection. The common result is, a party or sect is formed (1 Cor. 11:19; Gal. 5:20; 2 Pet. 2:1). One who adheres to a heresy is a heretic, and after the first and second admonition is to be rejected (Titus 3:10). God having given in His word all things needed for the church, there is no room for man’s choice or man’s will: he must be a humble receiver (compare 1 Cor. 4:7).
From Anstey’s Doctrinal Definitions:
This refers to an outward division in the Christian testimony, wherein a party separates itself as a distinct company. “Heresy” is the making of a “sect” among Christians and is translated as such in 1 Corinthians 11:19 in the J. N. Darby Translation. It is an evil that emanates from the flesh—the fallen sin-nature (Gal. 5:20). In Scripture, it is applied to divisions that had developed in the Jew’s religion (Acts 5:17; 15:5; 24:5; 26:5) and divisions that would develop in Christianity (1 Cor. 11:19; Gal. 5:20; 2 Peter 2:1).
Heresy is not the same as a “schism,” which is an inward split or rift among Christians (1 Cor. 11:18 – marginal reading in the KJV). Those involved in a schism will still meet outwardly with those with whom they differ, but probably unhappily (Rom. 16:17; 1 Cor. 1:10; 3:3; 11:18). This was the case with the Corinthians. The Apostle Paul warned them that if schisms existed, and were not dealt with and judged as evil, “there must also be heresies” that would grow out of those schisms (1 Cor. 11:18-19). Hence, an inward split (a schism) will develop into an outward split (a heresy), given enough time. A person who engineers an outward split among Christians is a “heretic” (Titus 3:10). Paul tells us that if we meet such a person who has gone out in self-will with his party, we are to, “after a first and second admonition, have done with him” (Titus 3:10).
Heresy is commonly thought to be bad doctrine, and was popularized as such by the Roman Catholic Church hundreds of years ago. They labeled everyone who would not uphold their doctrines as heretics. Bible teachers today sometimes use the term conventionally to indicate doctrinal error, but heresy, as to its Scriptural meaning, does not necessarily involve bad doctrine at all. G. V. Wigram tells us that the worst and most difficult kind of heresy to detect is that which does not involve bad doctrine, yet the spirit of party-making and division are there (Memorials of the Ministry of G. V. Wigram, vol. 2, p. 91). Heresy has probably become synonymous with bad doctrine because most heretics usually form their division around bad doctrine (2 Peter 2:1).
Thus, a heresy/sect is the forming of a fellowship of believers outside of, and separate from, the one fellowship to which all Christians have been called in Scripture—which “the Lord’s Table” symbolizes (1 Cor. 1:9; 10:21). The evil of heresy is that it divides the Christian testimony into separate groups and fellowships, each with their own administration, by-laws, etc. It is not God’s will to have Christians divided like this. He would have them to meet together on one ground of fellowship (even though they may be in various localities all over the world), so as to give expression to the fact that they are “one body” (Eph. 4:4). The making of all such churches in Christendom (“church planting” as it is called) is usually done with good intentions but in ignorance of God’s way of gathering Christians together for worship and ministry. We, therefore, need to show much patience with those who are in these man-made fellowships. Nevertheless, all such church making efforts destroy the unity of the body in practise.