Source: The Stone That Never Came Down (1973), Chapter 23 (p. 180)
“We then saw what St. Jerome said of those who serve God and those who serve the world: "Each to the other we seem insane": Invicem insanire videmur. There is a never-ending duel between the two.”
Mémoires pour servir à l'histoire de Port-Royal (1752), as cited by M. A. Screech in Laughter at the Foot of the Cross (1997), p. 69
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Jean De La Fontaine 47
French poet, fabulist and writer. 1621–1695Related quotes

“The world will, in the end, follow only those who have despised as well as served it.”
The World
The Note-Books of Samuel Butler (1912), Part XXIV - The Life of the World to Come

“God's commandment is that those who serve Him must separate themselves from the world.”
Source: Separation from the World, p. 4

McCreary County v. American Civil Liberties Union, 545 U.S. 844 (2005) (concurring).
Context: Reasonable minds can disagree about how to apply the Religion Clauses in a given case. But the goal of the Clauses is clear: to carry out the Founders’ plan of preserving religious liberty to the fullest extent possible in a pluralistic society. By enforcing the Clauses, we have kept religion a matter for the individual conscience, not for the prosecutor or bureaucrat. At a time when we see around the world the violent consequences of the assumption of religious authority by government, Americans may count themselves fortunate: Our regard for constitutional boundaries has protected us from similar travails, while allowing private religious exercise to flourish. [... ] Those who would renegotiate the boundaries between church and state must therefore answer a difficult question: Why would we trade a system that has served us so well for one that has served others so poorly?

The Voluntary Movie Rating System (2004)
Context: We count it crucial to make regular soundings to find out how the public perceives the rating program, and to measure the approval and disapproval of what we are doing... The rating system isn't perfect but, in an imperfect world, it seems each year to match the expectations of those whom it is designed to serve — parents of America.

For My Legionaries: The Iron Guard (1936), Jewish Problem

“Religions are manipulated in order to serve those who govern society and not the other way around.”
"Sex Is Politics" (1979)
1980s, The Second American Revolution (1983)

“A duel is just two murderers who agree to take turns trying to kill each other.”
Source: The Tales of Alvin Maker, Seventh Son (1987), Chapter 11.