“What is too absurd to believe is believed because it is too absurd to be a lie.”

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.

Source: The Janson Directive

"The Three Fish" Ch. 18 : The Three Fish, p. 196
Disputed, The Essential Rumi (1995)

Source: Here Be Dragons: Telling Tales Of People, Passion and Power

Source: Justine

Possibly a paraphrase of Bertrand Russell in My Philosophical Development (1959): "This is one of those views which are so absurd that only very learned men could possibly adopt them." It is similar in meaning to Orwell's line from Notes on Nationalism (1945): "One has to belong to the intelligentsia to believe things like that: no ordinary man could be such a fool." However, Russell was commenting not on politics, as Orwell was, but on some philosophers and their ideas about language.
Misattributed
Variant: Some ideas are so stupid that only intellectuals believe them.

Quoted in "Believe, Obey, Fight" - Page 98 - by Tracy H. Koon - Political Science – 1985.

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