To make someone adopt heathen beliefs or practices, or to convert to a non-religious or non-Christian way of life.
From 'heathen' (Old English hǽthen, meaning 'not Christian') plus the suffix '-ise' (British spelling of '-ize'), which comes from Greek '-izein'. The verb form emerged in the 16th-17th centuries to describe the process of becoming or making something heathen.
This word captures a fascinating moment in English history when Christian Europe was actively converting populations and was deeply concerned with the reverse process—people 'falling away' from Christianity. The very existence of this word shows how central Christian identity was to European self-understanding.
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