Not To Be Confused With
Heresy is false DOCTRINE; schism is broken COMMUNION (a split in the Church's unity). One can occur without the other, though they sometimes overlap.
From Greek hairesis, “a choosing” — originally a faction or “party” (the word for the Sadducees' “sect” in Acts). It came to mean a self-chosen teaching that breaks with the received faith on something central: denying the deity of Christ, the Trinity, the resurrection, salvation by grace.
The weight of the word matters. Heresy is not any mistake or any view you dislike; it is the denial of a defined essential — the kind of error the ecumenical councils named and excluded. Calling a fellow Christian's debatable opinion “heresy” is itself a misuse of the term.
Examples
- Arianism — denying that the Son is fully God.
- Docetism — denying that Christ truly became human.