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• Abandoner of the cause
• Backslider
• Cause abandoner
• Cause forsaker
• Defector from the faith
• Deserter
• Forsaker of the faith
• Heretic
• Julian the___
• One who has lapsed
• One who loses faith
• One who's fallen
• Party switcher, say
• Religion renouncer
• Religious defector
• Renegade
• Renouncer of one's faith
• Turncoat
• A disloyal person who forsakes his cause or religion or political party or friend etc.