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In the 19th century , Nietzsche began to write a series of polemics on the " unnatural " teachings of Christianity ( e.g. sexual abstinence ) , and continued his criticism of Christianity to the end of his life .
 Nietzsche
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Nietzsche