Definition: Heresy is a departure from the truth.
In the early history of Christianity, the church established the basic teachings of the faith. Those basics can be found in the Apostles' Creed and Nicene Creed. Over the centuries, however, theologians and religious figures have proposed doctrines that contradict established Christian beliefs. To keep those beliefs pure, the church singled out people who taught or believed ideas considered a threat to Christianity.
It wasn't long, however, before so-called heretics were branded not only as enemies of the church but also as enemies of the state. Persecution became widespread as popes authorized inquisitions. Those investigations often resulted in torture and execution of innocent victims. Thousands of people were imprisoned and burned at the stake.
Most heresy proposes views of Jesus Christ and God that are contrary to what is found in the Bible. Heresies include Gnosticism, modalism (the idea that God is one person in three modes), and tritheism (the idea that the Trinity is actually three separate gods).
Pronunciation: HAIR uh see
Example: Judaizers promoted a heresy that said Gentiles had to become Jews before they could become Christians.