Scifag is a derogatory term for a person obsessed with Scientology.
The word 'scifag' is a portmanteau on the words 'Scientology' and 'fag
', the latter being short for 'faggot
', a derogatory term for homosexuals. The wording of (noun)-fag carries the meaning of being 'gay for (noun)', which in turn means 'to be obsessed with (noun)', e.g. a Gucci-fag is a person obsessed with Gucci products.
The word scifag therefore describes someone who is 'gay for Scientology', meaning they are obsessed with it. It is likely that it was derived from a similar, pre-existing term also about a religion, namely Cristfag
(someone gay for Jesus Christ, and in extension, Christianity)