Heresy:

A teaching or belief that is contrary to the belief of the Catholic Church

Heretic:

A person believing in or practicing religious heresy

1 John 4:1: Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world.

Catechism 2104: 26 "All men are bound to seek the truth, especially in what concerns God and his Church, and to embrace it and hold on to it as they come to know it."