idioms - Definite or indefinite article in "the/a devil's advocate"

I can't quite figure out which of the following expressions is more correct:

• He is the devil's advocate.


• He is a devil's advocate.


• He is playing devil's advocate.

The combination of an article with the possessive is what confuses me. Exactly which word(s) does the article apply to?

The first form seems to suggest either that he is an advocate of The Devil -- namely, Satan himself -- or even worse, that he is The Advocate of The Devil. (Kill him with fire!)

The second form seems to suggest that he is an advocate of a devil (but not necessarily of The Devil, nor the only advocate out there.) This seems to fit better with the way this idiom is commonly used, but I haven't seen this idiom used very often with the indefinite article. It's usually used with the definite article.

The third form suggests that he is playing a role named "devil's advocate", with no article attached to it.

Similar examples: The King's speech, the Indian's prayer, the mother's room, etc.

Answer

He is the Devil's advocate.

This is the classical expression. The term was used by the Catholic Church (from 1587 until the office was abolished, in 1983) for the canon lawyer who was supposed to argue against proposals for canonisation, i.e. adding someone to the official list of saints, the canon. The purpose of these arguments against canonisation was to test the strength of the arguments for canonisation as brought forward by God's Advocate.

Syntactically, it is ambiguous whether the modifies Devil or advocate; however, in this case it must modify Devil. That is because the Devil normally requires the definite article if you are referring to the one and only Christian Devil, which is the case here. The definite article can sometimes be left out, but that would be ellipsis; in that case, however, advocate shouldn't have an article either, because Devil requires it while advocate doesn't. Compare the following sentence:

He is Cleopatra's advocate.

He is the Queen's advocate.

Being someone's x usually doesn't require an article before x. Whose advocate is he? The Devil's! It would be odd to add the article where it is normally left out (with advocate) while omitting it where it is normally used (in the Devil).

He is a devil's advocate.

The indefinite article sounds less idiomatic. The article the as above could be left out in casual use; but then it would sound odd to use a phrase almost identical to the full classical expression the Devil's advocate, having merely swapped one article for the other. If you mean to say that a specific person answers to this description, use the; if you were mentioning the general concept of being a devil's advocate, you could very well use a.

He is playing devil's advocate.

Here the article is dropped in a casual manner, and the phrase is used loosely in a slightly changed environment: this is how the phrase is most often used.