heretic

If your friend became interested in Hinduism, with its many gods and rituals, her Catholic mother might be worried that her daughter was a heretic, or a person whose religious beliefs are in contrast to the fundamental beliefs of her church.

The noun heretic is mostly used in a religious context to talk about someone whose actions or beliefs act against the laws, rules, or beliefs of some specific religion. However, heretic can also be used in a non-religious way to mean "someone whose ideas go against the norm." Say your regular hangout is a cowboy honky-tonk and most of your friends play in country bands; you might be considered a heretic if you announced that your favorite kind of music is actually opera.