The main definitions of the noun anathema are (1) a detested person or thing, and (2) a formal ecclesiastical ban. The term comes directly from Latin, where it meant a doomed offering. It is most often used to denote someone or something that is reviled by a particular group. It conventionally functions a mass noun, so it does not take an article. For example, he is anathema is more conventional than he is an anathema and he is the anathema.
Anathema is widely misused, sometimes as a noun and sometimes adjectivally, to describe obstacles, opposites, or things that oppose. For example, these writers use the word in ways that don’t correspond to its conventional definition:
[B]eing feminine in the workplace was anathema [an obstacle?] to getting ahead. [NJ.com]
Officialdom has always seemed anathema to [to oppose?] contemporary art, which seems to thrive in adversity. [Toronto Star]
But to deny one class of citizens the freedom to marry based on their sexual orientation is discriminatory – anathema [in opposition to?] to the great tradition of freedom and equal opportunity … [New York Daily News]
And these writers correctly use anathema to refer to something that is detested:
But that does mean that sometimes drivers have to slow down, which is anathema to many Americans. [Forbes]
He has had to account for his former support for a cap-and-trade anti-pollution plan, which is anathema to conservative voters. [Telegraph]
But even if that wasn’t the case, the concept of being idle is anathema to him. [Sydney Morning Herald]