a person who has deserted their cause
apostate - deserter
An ‘apostate’ is somebody who has abandoned their faith and allegiance to a particular religion, party, or cause. Even if the act itself is done for good reason and with the best of intentions, the reaction from the group that is left behind will still invariably be hostile as they tend to view such people as traitors. This formal word, therefore, can have both positive and negative connotations depending on the context.
The politician became an apostate when she switched to a rival party before a major vote. She said she was simply following her conscience.
Even though he still considers himself a practising member of that religion, the writer was accused of apostasy after calling for urgent religious reforms and modernisation.
The organization does not allow for any diversity of opinion. Anybody who refuses to toe the official line is considered an apostate.