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Could you enlighten me as to the usage of the twin expressions "persona grata" and "persona non grata"? Can they be used in the plural? Are they restricted to diplomatic circles alone?
, especially "formal and rarely used". In the plural I've only ever met it in the context of diplomacy. In the singular it's slightly more widely used, but still formal. If a man is told by his wife 'Get out - you're not welcome any more' he might report the words in a formal deposition (e.g. sworn statement to be presented in divorce proceedings) as 'She said I was persona non grata'. But most people can get through life without using the phrase at all.
Dear BobK
Is it correct to say " He was declared persona non grata and banished from the country"? Or should we say "He was declared a persona non grata and...
The noun appears to be singular in from and hence the question. Please comment.
You don't need the "a". Latin had no system of articles, so persona could mean either "person" or "a person" or "the person" depending on context.
That said, it's a question of where you draw the line between English and Latin - "he was declared a [English] persona non grata [Latin]..." or "he was declared [English] persona non grata [Latin]..." For many users "persona non grata" is fully Anglicized, and in that case it would make sense to use the article. I've heard both.
persona grata 
, especially "formal and rarely used". In the plural I've only ever met it in the context of diplomacy. In the singular it's slightly more widely used, but still formal. If a man is told by his wife 'Get out - you're not welcome any more' he might report the words in a formal deposition (e.g. sworn statement to be presented in divorce proceedings) as 'She said I was persona non grata'. But most people can get through life without using the phrase at all. 
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