“Do not believe in an absurdity
no matter who says it.”

Source: All I Really Need to Know I Learned in Kindergarten

Sebban Balwer
(15 October 1994)
Source: Lord of Chaos

CW 12, par. 126 (p 99)
Psychology and Alchemy (1952)
Context: People will do anything, no matter how absurd, in order to avoid facing their own souls. They will practice Indian yoga and all its exercises, observe a strict regimen of diet, learn the literature of the whole world - all because they cannot get on with themselves and have not the slightest faith that anything useful could ever come out of their own souls. Thus the soul has gradually been turned into a Nazareth from which nothing good can come.

Source: Around the Day in Eighty Worlds

"The Absurd" in Mortal Questions, Cambridge University Press, 1979, p. 23.

Preface (p. xiii; quoting Voltaire)
Irreligion: A Mathematician Explains Why the Arguments for God Just Don’t Add Up (2008)

Variant translations
It is by all means to be believed, because it is absurd.
It is is entirely credible, because it is inept.
It is certain because it is impossible.
De Carne Christi 5.4
Often paraphrased or misquoted as "Credo quia absurdum."
Also paraphrased as "It is so extraordinary that it must be true."
Two lines from De Carne Christi have often become conflated into the statement: "Credo quia impossibile" (I believe it because it is impossible), which can be perceived as a distortion of the actual arguments that Tertullian was making.

Plato, Phaedo

Nothing Created Everything: The Scientific Impossibility of Atheistic Evolution (2009)