a follower of Donaldism.
For example, the D.O.N.A.L.D. ("Deutsche Organisation der Nichtkommerziellen Anhänger des Lauteren Donaldismus" - German organization of noncommercial followers of undiluted Donaldism) is a society for the scientific analysis of the Barks/Fuchs Disney comic universe, established 1976.
Donaldists go as far as sneaking into respectable newspapers and having Erika Fuchs quotes pop up in the arts supplement, undetected for more than 10 years.