Synonyms of 'backslider'
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An apostate is someone who has abandoned their religious faith, political loyalties, or principles.
He was an early apostate, leaving the party last year.
A deserter is someone who leaves their job in the armed forces without permission.
He was a deserter from the army.
A renegade is a person who abandons the religious, political, or philosophical beliefs that he or she used to have, and accepts opposing or different beliefs.
He was a renegade - a traitor.
Thesaurus for backslider from the Collins English Thesaurus