Apostate is a noun that refers to a person who abandons a group for another allegiance.

The Greek word aphistastha (ah FEAST uh thigh) means ‘revolt.’ Early on, it was used mainly in a religious context to refer to someone revolting against a particular church or religion. In more recent years,  it’s used to refer to someone who abandons any kind of group or organization to make an allegiance with another.

I felt a sense of shock and betrayal when I discovered that Riley had left our band and joined the other polka band in town. As far as I’m concerned, he’s the most outrageous apostate I’ve ever encountered.