For example, when used as a predicate nominative, anathema doesn't require an article: That belief is anathema. However other words do require an article: He became a knight.
I wonder if it has something to do with singularity of the predicate nominative: I will be king.
The only research I have done is to look up definitions for predicate, nominative, copulatory verbs, as well as googling to try to find reasons why anathema could not require and article. This was difficult to find as anathema could be counted, depending on the definition in use.