A heresy is a sect. A sect is a party formed for the purpose of separating some of God's children from the rest. Sects are formed by factionalists, those under the insidious influence of the party spirit. No one who is honestly mistaken about some matters of scriptural interpretation is a heretic. To be a heretic one must make a test of fellowship out of his opinion or interpretation and attempt to establish a party to promote or protect that view. Not only is the common view of 'heresy' held by the majority unscriptural, but it is inimical to the peace and harmony of the body of Christ, for many reasons.
1) It brands and stigmatizes humble seekers after truth whose character is above reproach and whose only crime is that they cannot concur in every view of opinion held by those who have assumed the role of infallible interpreters of the sacred scriptures.
2) It assumes that each faction or party has an infallible interpretation of the word of God at the same time that it denies the possibility of an infallible interpreter.
3) It makes real communication and interchange of ideas with other sincere students of the word in the Christian realm virtually impossible.
4) It breeds inconsistency of the worst kind. We are betrayed into rejecting those who have not attained unto a certain intellectual status at the same time that we receive those whose moral and ethical behavior is inferior. It rejects those whom God receives and receives those whom God rejects.
5) It denies the validity of the only law bound upon those in Christ Jesus--the law of love. It limits and restricts the real applicability of this law to those who conform to the party norm, and thus reduces it to a factional dispensation. If you doubt this, all you need to do is to read the various journals to learn that each has its own circumscribed 'brotherhood' and each of these brotherhoods is composed of those who conform to the party test.
(The brief comments above are from an article entitled "What Is Heresy?" by W. Carl Ketcherside. The article is contained in a journal, Sound Words, that was published in the mid 1980's.)