Ideologue(noun)
An adherent of an ideology, especially one who is uncompromising and dogmatic.
Hindi meaning:विचारक
Synonyms: Believer, Adherent, Advocate, Backer (Of A Cause)
Antonyms: Antagonist (Of A Cause)
Example: When Jeff’s governmental ideology changed, he decided to start his own political party.
Trick to remember: India me alag alag idea se IDEOLOGUE ideology batate hai 
