IPA: //ʌnˈbɪliːvər//
KK: /ʌnˈbɪlivər/
A person who does not have faith or belief in something, especially in a religion.
He was considered an unbeliever because he did not follow any religious practices.
Unbeliever → It is formed from "un-" (meaning not) and "believer" (from the root "believe" which comes from Old English "beliefe", meaning to have faith or trust). The word "unbeliever" means a person who does not have faith or trust in something, particularly in a religious context.
Think of 'not' ('un-') having 'faith' ('believe') — that's why an unbeliever is someone who does not have faith.