Defined: Heresy is the formal denial or doubt of any defined doctrine of majority religion.
The saint of one church is the heretic of another. So who decides what a heresy is? Splits have occurred in the Christian faith as well as other faiths because of differences of opinion over doctrine. For example after the New Testament Christian church became legal in the
Empire about 325 AD, The church in the western part began using
statues as objects in worship. At the same time the church in the eastern part
began using pictures or icons in their worship. The Eastern Orthodox claimed
that the Catholics were idol worshipers and the Roman church claimed the same
about the Eastern Orthodox.
Protestant and Roman Catholic both made similar claims about each other. Today there are over 350 Christian denominations with many claiming to the only true church with all the rest being in heresy. Human pride, stubbornness, and traditions come between us.
We are not alone. There are three main branches of the Jewish faith and at least that many of the Moslem faith. The Buddhist have also split in the same fashion. We all decide that we are right and every one else is a heretic and then fight about it. So sad.