The weak cannot be sincere.

[Les personnes foibles ne peuvent être sincères.]

François VI, duc de La Rochefoucauld (1613–1680) French epigrammatist, memoirist, noble

Réflexions ou sentences et maximes morales [Reflections; or Sentences and Moral Maxims], ¶316 (1665-1678) [tr. Tancock (1959)]
    (Source)

First included in the 3rd (1671) edition. In manuscript it read, "Les gens foibles ne sauroient avoir de sincérité."

(Source (French)). Other translations:

Half-witted People can never be sincere.
[tr. Stanhope (1694), ¶316]

Weak people are incapable of sincerity.
[pub. Donaldson (1783), ¶475; ed. Carvill (1835), ¶403; tr. FitzGibbon (1957), ¶316]

Weak persons cannot be sincere.
[ed. Gowens (1851), ¶326; tr. Bund/Friswell (1871), ¶316]

A weakling is incapable of sincerity.
[tr. Heard (1917), ¶323]

Sincerity is incompatible with weakness of character.
[tr. Stevens (1939), ¶316]

Weak people cannot be sincere.
[tr. Kronenberger (1959); Whichello (2016), Chandler; all ¶316]