St. Francis de Sales

True and living friendship cannot endure in the midst of sin. It is said that the salamander extinguishes the fire in which it lives, and sin ruins the friendship in which it lodges itself. If it is just a passing sin friendship puts it to flight at once by correction. But if sin stays and settles down, immediately friendship perishes because it can only be kept up by true virtue. Then, how much less should we sin for the sake of friendship!

- Introduction to the devout life