"Le bon David" meets 15 atheists.

During his period in Paris as secretary to the English ambassador Lord Hertford  between 1763 and 1766  David Hume was lauded by fashionable Parisian society.  This famous anecdote, told in a letter of Diderot's, relates to his first appearance as a guest of the baron d'Holbach:.

Diderot to Sophie Volland 6 October 1765:
The first time that M. Hume found himself at the table of the baron [D'Holbach], he was seated beside him. I don't know for what purpose the English philosopher took it into his head to remark to the Baron that he did not believe in atheists, that he had never seen any. The Baron said to him: "Count how many we are here." We are eighteen. The Baron added: "It isn't too bad a showing to be able to point out to you fifteen at once: the three others haven't made up their minds"
http://fr.wikisource.org/wiki/Lettres_%C3%A0_Sophie_Volland/9

Diderot later told the same story to Samuel Romilly: Hume and d'Holbach were seated together and discussing natural religion when Hume made his remarks.
https://books.google.co.uk/books?id=6zcNAAAAYAAJ&pg=PA131#v=onepage&q&f=false


What did Hume mean?

Why did Hume, in the most radical freethinking company in Europe, suddenly decide to declare that he "did not believe in atheists"?  Diderot himself seemed puzzled, and much ink has been spilled on the question since.  Surely Hume could not really have intended to deny the possibility of speculative atheism?  It seems much more likely, as David Berman suggests,  that Hume was deliberately trying to provoke a response from d'Holbach;  his second assertion, that he had never seen any atheists, clearly invited the baron's triumphant retort. "Hume's opening gambit was rather like a Masonic handshake: an attempt to elicit a response from, and communicate with, someone whose secret identity he guesses" (Berman, p,63)  Hume certainly remained on very good terms with both Diderot and d'Holbach and corresponded familiarly with them after his return to London in 1766;  Diderot addressed his letters to the "well-beloved and greatly honoured David".  Hume later related to Boswell that he had similarly provoked Lord Marischal, "a downright atheist", who had not spoken to him for a week when he "hinted something as if I believed in the being of a God".

La lecture des philosophes (Collection Jean-Jacques Monney, Genève)

Were there really fifteen atheists?

D'Holbach's words too should not be taken too literally.  Alan Charles Kors in his classic study of the coterie d'Holbach  thoroughly debunked the idea that the baron's gatherings were hotbeds of atheism: only d'Holbach himself and Naigeon truly conformed to the stereotype of the proselytising atheist, whilst a mere handful of other members -    Diderot, Helvétius, perhaps Augustin Roux - seriously espoused atheism as a philosophical system (Kors, p.63).  D'Holbach, however, clearly relished his bon mot and signalled effectively to his guest that "atheism" was not only a permitted subject of discussion at his dinners but one which was actively encouraged.

Hume and d'Holbach - portraits by Carmontelle

References

David Berman, "Hume's atheism" in A History of atheism in Britain: from Hobbes to Russell (1990) p.101-2
https://books.google.co.uk/books?id=g7L1aY0ULiYC&pg=PA101#v=onepage&q&f=false

Alan Charles Kors, D'Holbach's coterie: an Enlightenment in Paris (Princeton UP, 1976).  See particularly p.62-3.
https://books.google.co.uk/books?id=m_l9BgAAQBAJ&dq=kors+coterie+d%27holbach&source=gbs_navlinks_s

Hume was intellectually much more in sympathy with the baron's undecided guests:
See Robert Zaretsky, "Hume and humility" Engines of our Ingenuity
http://www.uh.edu/engines/epi2173.htm