Heresy = belief opposed to orthodoxy
Heresiarch = founder of a heresy
Heretic = a follower of heresy
We don't like to talk about heresy today. It sounds so archaic, judgmental and intolerant. Yet Paul did in Galatians 1:6-7. He was shocked that the Christians in Galatia were "so quickly deserting the one who called you by the grace of Christ into a different gospel which is not another except certain ones are disturbing you and wanting to distort the gospel of Christ."
The root of heresy is deserting a person for a message. The Galatians turned away from "the one who called you" (τοῦ καλέσαντος ὑμᾶς) "to another gospel" (εἰς ἕτερον εὐαγγέλιον). The preposition "to" (εἰς) suggests mental movement in a new direction with the intention to accept a new doctrine. For example, the preposition is used to describe the movement of unbelievers coming to Christ in Acts 26:18. (Burton, Galatians, 22). Here in Galatians 1:6 we see the opposite mental movement. Heresy is turning away from Christ to another gospel.
Paul uses two different pronominal adjectives when he writes "to another (ἕτερος) gospel which is not another" (ἄλλος). These two adjectives are frequently used interchangeably without any distinction of meaning (Blass/Debrunner, Grammar, p.161). It is possible that Paul intended the change of adjectives in Galatians 1:6-7 merely for stylistic variation. However, Robertson insists that Paul intended a real difference between the two in this context (Robertson, Grammar, 747). Paul admits that they are preaching "another" (ἕτερον) gospel different from his but he rejects the idea that they are preaching "another" (ἄλλο) gospel like his. A common distinction that writers make (Moulton, Grammar, 1:246) is between "another of a different kind" (ἕτερον) and "another of the same kind" (ἄλλο).
I think Paul intends a distinction between the two adjectives in these verses so let's mine the matter a little deeper. The adjective ἕτερος indicates one of two while ἄλλος implies one beside another. The first adjective (ἕτερος) distinguishes while the second adjective (ἄλλος) adds (Lightfoot, Galatians, 76). The first implies a substitute and the second involves an addition. Heresy can take two forms. Sometimes heresy is an alternative to orthodoxy. Sometimes heresy is an addition to orthodoxy. An alternative is easier to identify because it is unlike truth. It replaces truth. An addition is more subtle because it implies that the truth is not replaced by the new doctrine. The new teaching merely adds to the truth. However, both are heresy as Paul makes clear.
An alternative gospel rejects the apostolic gospel. The apostolic preaching of the cross is wrong. Down through church history, many have preached alternative gospels. The "moral influence theory," the "example theory," and the "governmental theory," of the atonement are alternative gospels. The substitutionary death of Christ is in error according to alternative gospels. Jesus didn't die on the cross to satisfy God's wrath for our sins. He was a good man who died a tragic death as an example. He showed us how to live and die. An alternative gospel is heresy, a turning away from Christ to preach a different message than Christ's message.
An additional gospel argues that the original gospel is true but old fashioned. We don't deny the apostolic gospel, but we need to update it for today. We need to add elements to make the archaic message relevant to people. Preachers of an additional gospel want to freshen it up for a modern world without denying what the apostles taught. They want to add to without subtracting from the preaching of the cross. For example, the blood of Christ is offensive to the modern mind so let's not talk or sing about a bloody cross. It's true, but we won't mention it. An additional gospel argues that preaching grace alone is insufficient for salvation so we will add works or rituals that people must do to earn God's favor.
Paul quickly disabuses us of any notion that an additional gospel is not heresy. He says that an additional gospel is not an addition at all. Paul uses an exception clause to make this clear. He writes that it is not an additional gospel except (εἰ μή) in the sense that false teachers are agitating you and distorting the gospel of Christ (Burton, Galatians, 24). An additional gospel "adds" elements that distort the true gospel making it no gospel at all.
The good news becomes bad news as much by distortion as by replacement. Whether by addition or subtraction, heresy is still heresy!