The first sign of corrupt morals is the banishing of truth.

[Le premier traict de la corruption des mœurs, c’est le bannissement de la verité]

Michel de Montaigne (1533-1592) French essayist

Essays, Book 2, ch. 18 (2.18), “Of Giving the Lie [Du Démentir]” (1578–79) [tr. Screech (1987)]
    (Source)

This essay (and this passage) appeared in the 1st (1580) edition, and was expanded in each succeeding edition.

(Source (French)). Alternate translations:

The first part of customs-corruption, is the banishment of truth.
[tr. Florio (1603)]

The first step to the corruption of manners is banishing of truth.
[tr. Cotton (1686)]

The first thing done in the corruption of manners is banishing truth.
[tr. Cotton/Hazlitt (1877)]

The first feature of corruption of morals is the banishment of truth.
[tr. Ives (1925)]

The first feature in the corruption of morals is the banishment of truth.
[tr. Zeitlin (1934)]

The first stage in the corruption of morals is the banishment of truth.
[tr. Frame (1943)]