― Mais quelle est votre attitude maintenant vis-à-vis de la religion?
― Je n’ai pas d’attitude. J’ai été élevé dedans. Je pourrais répondre «Je suis toujours athée, grâce à Dieu.» Je crois qu’il faut chercher Dieu dans l’homme. C’est un attitude très simple.

☛ L’Express: “Luis Buñuel: athée grâce à Dieu” by Michèle Manceaux, May 12, 1960, p. 41.

That’s the original form of the quote known in English as “Thank God I’m an atheist”. Here’s an English translation of the original exchange between Luis Buñuel and Michèle Manceaux (as quoted above):

― But now what is your attitude towards religion?
― I have no attitude. I was raised in it. I could answer “I’m still an atheist, thank God.” I believe we must seek God within man himself. This is a very simple attitude.

Seventeen years later, Luis Buñuel had a different attitude toward the subject of religion. He discussed it during an interview he gave for The New Yorker:

Buñuel’s work is born of the humor of a sad man distressed by his own vision of the world but with a fond eye for the mass of self-deceptions that make life bearable. He is one of the most mannerly men I have ever met. The only thing that stirs him to impatience is the sort of charity that is seen as a virtue by people who don’t question the anguish that makes charity necessary. “I’m not a Christian, but I’m not an atheist either,” he says. “I’m weary of hearing that accidental old aphorism of mine ‘I’m not an atheist, thank God’ It’s outworn. Dead leaves. In 1951, I made a small film called ‘Mexican Bus Ride,’ about a village too poor to support a church and a priest. The place was serene, because no one suffered from guilt. It’s guilt we must escape, not God.” (The New Yorker: “Long Live the Living!” by Penelope Gilliatt, December 5, 1977, p. 54)

There’s most likely a small confusion when Buñuel quotes his own aphorism: instead of “I’m not an atheist, thank God” ―which doesn’t correspond what he said in 1960― one should read “I’m an atheist, thank God”. That’s the “aphorism” Buñuel didn’t agree with anymore in 1977.