Sentence:  Traditionalists see only campaigns launched to recover Jerusalem as true crusades; generalists argue that any Christian war fought for God was a crusade; <WSD>popularists</WSD> claim that crusading came out of popular, peasant movements; while pluralists argue that any war in which the participants took a vow and gained spiritual rewards could be seen a crusade. 
Meaning of the word marked with <WSD> tags:  One who explains social phenomena in terms of popular responses and habits.