Nothing is so firmly believed as what is least known.
 
[N’est rien creu si fermement, que ce qu’on sçait le moins.]

Michel de Montaigne (1533-1592) French essayist

Essays, Book 1, ch. 31 “We Should Meddle Soberly with Judging Divine Ordinances [Qu’il faut sobrement se mesler de juger des ordonnances divines]” (1572) (1.31) (1595) [tr. Frame (1943), ch. 32]
    (Source)

Both the essay and the quote appeared in the 1st (1580) edition.

(Source (French)). Alternate translations:

Nothing is so firmely beleeved, as that which a man knoweth least.
[tr. Florio (1603)]

Nothing is so firmly believed as what we least know.
[tr. Cotton (1686)]

What we know is the least of what we do not know.
[tr. Friswell (1868)]

Nothing is so firmly believed, as what we least know.
[tr. Cotton/Hazlitt (1877)]

Nothing is so firmly believed as that which we know least.
[tr. Ives (1925), ch. 32]

Nothing is so firmly believed as whatever we know least about.
[tr. Screech (1987), ch. 32]