Communist leader followed popular athiest (7)
I believe the answer is:
infidel
But I can't explain it at all! Could you help me understand?
(Other definitions for infidel that I've seen before include "Pagan" , "Unbeliever" , "One who doesn't believe" , "Person who rejects a religious belief" , "Literally, 9 [UNFAITHFUL]" .)