Cthulhu: only one cultist and blow the head
 

If I'm the only cultist in the game can someone play sudden blow to the head on me and I lose my cultist class?

The rules say nothing but divine intervention can make a cultist who is the only cultist in the game stop being a cultist.

I don't understand if that takes precedence over cards or if it's supposed to follow as a consequence from other rules (so cards take precedence).

Re: Cthulhu: only one cultist and blow the head
 

Nope. A card would have to say "Lose your Cultist, even if you are the only one" to override that rule.