Often used when someone abandons their religion, seen negatively by that religious community. After converting to Buddhism, his family labeled him an apostate.
Pro Tip 2/3
Secular Context
Can also refer to leaving non-religious beliefs, like political ideologies. She was an apostate of the party's ideology after the scandal.
Pro Tip 3/3
Cultural Sensitivity
Be aware of religious or cultural sensitivity when using, as it can offend. Calling him an apostate in public led to social tensions.