"Heretic" means divider or one who causes schism. We think of modern heresy-hunters as those who sniff out doctrinal error and expose it. But Biblically speaking, heretics are those who bring needless division. They use a magnifying glass to discover blemishes, or points of disagreement, in and with others.
"Where the devil does not go personally, he sends a critic", said a wise Puritan. Every church and every movement has its critics. Detractors never offer an unqualified word of approval. Unlike metal, their tongues get sharper after every jab. Being unsympathetic is one thing, but having a perennial critical spirit is another.