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He cites Bernard of Clairvaux " his father " , very frequently , while much less frequently Augustine of Hippo and Gregory the Great are quoted .
 Bernard, Clairvaux, Augustine of Hippo, Gregory the Great
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Bernard, Clairvaux, Augustine of Hippo, Gregory the Great