What makes one friend sure of another is the knowledge of his integrity
 Étienne de La Boétie, Discourse on Voluntary Servitude (1576). copy citation

add
Author Étienne de La Boétie
Source Discourse on Voluntary Servitude
Topic integrity self-knowledge
Date 1576
Language English
Reference
Note Translated by Harry Kurz
Weblink https://en.wikisource.org/wiki/Discourse_on_Voluntary_Servitude

Context

“Friendship is a sacred word, a holy thing; it is never developed except between persons of character, and never takes root except through mutual respect; it flourishes not so much by kindnesses as by sincerity. What makes one friend sure of another is the knowledge of his integrity: as guarantees he has his friend's fine nature, his honor, and his constancy. There can be no friendship where there is cruelty, where there is disloyalty, where there is injustice. And in places where the wicked gather there is conspiracy only, not companionship:” source