It's from the "unfortunate gathering" at the beginning of Brothers Karamazov.
Fyodor Karamazov speaking to Father Zossima:
Quote:
“…I’m like the philosopher, Diderot, your reverence. Did you ever hear, most Holy Father, how Diderot went to see the Metropolitan Platon, in the time of the Empress Catherine? He went in and said straight out, ‘There is no God.’ To which the great bishop lifted up his finger and answered, ‘The fool has said in his heart there is no God'. And he fell down at his feet on the spot. ‘I believe,’ he cried, ‘and will be christened.’ And so he was. Princess Dashkov was his godmother, and Potyomkin his godfather.”
“Fyodor Pavlovitch, this is unbearable! You know you’re telling lies and that that stupid anecdote isn’t true. Why are you playing the fool?” cried Miusov in a shaking voice.
“I suspected all my life that it wasn’t true,” Fyodor Pavlovitch cried with conviction. “But I’ll tell you the whole truth, gentlemen. Great elder! Forgive me, the last thing about Diderot’s christening I made up just now. I never thought of it before. I made it up to add piquancy. I play the fool, Pyotr Alexandrovitch, to make myself agreeable. Though I really don’t know myself, sometimes, what I do it for. And as for Diderot, I heard as far as ‘the fool hath said in his heart’ twenty times from the gentry about here when I was young. I heard your aunt, Pyotr Alexandrovitch, tell the story. They all believe to this day that the infidel Diderot came to dispute about God with the Metropolitan Platon….”