An article asking “Are we living in a computer simulation made by super-intelligent post-humans?” (link included as a courtesy, but I wouldn’t waste your time) prompted me to look up the saying often attributed to G.K. Chesterton as:
When a man stops believing in God, he doesn’t believe in nothing – he believes in anything.
It’s never easy to track down the source of famous-but-lightly-sourced quotations on the web, because so many of the search results are from unsourced collections of quotations. But a bit of digging elicited this, which gives the correct citation as being Émile Cammaerts’ book, The Laughing Prophet: The Seven Virtues and G.K. Chesterton.
Cammaerts is discussing a famous passage from Father Brown in which the priest-detective asserts that “the first effect of not believing in God that you lose your common sense.” Cammaerts writes:
‘It’s drowning all your old rationalism and scepticism, it’s coming in like a sea; and the name of it is superstition.’ The first effect of not believing in God is to believe in anything: ‘And a dog is an omen and a cat is a mystery.’
The origin of “Chesterton’s” saying would thus appear to be this sentence (which some readers have apparently mistaken for a continuation of Cammaerts’ citations from Chesterton):
The first effect of not believing in God is to believe in anything.
The Yale Book of Quotations suggests that Cammaerts’ formula may be a blend of two statements from the Father Brown books (the first being from the passage which Cammaerts discusses above):
It’s the first effect of not believing in God that you lose your common sense. [“The Oracle of the Dog” (1923)]
You hard-shelled materialists were all balanced on the very edge of belief – of belief in almost anything. [“The Miracle of Moon Crescent” (1924)]