No. The word ‘atheist’ is not a proper noun. It’s just a regular noun.
A ‘noun’ is any word that refers to a thing – an object. ‘Book’, ‘pen’, ‘table’, ‘apple’ are all examples of nouns – they are all things. A ‘proper noun’ is any word that refers to a specific thing – it’s the name for that thing. For example, the word ‘city’ is a noun, but the word ‘London’ is a proper noun, as it refers to a specific city – not just any city. Similarly, the word ‘language’ is a noun, but the word ‘French’ is a proper noun – as it refers to a specific language – it is the name of a language.
In English, proper nouns are always capitalised, no matter where they appear in a sentence, whereas regular nouns generally aren’t capitalised (unless they appear at the start of a sentence).
The word ‘atheist’ is not a proper noun, so it does not need to be capitalised.