“Zeal to do all that is in one's power is, in truth, a proof of piety.”

— Julian (emperor)

As quoted in The Works of the Emperor Julian (1923) by Wilmer Cave France Wright, p. 311; also in The Paganism Reader (2004) edited by Chas S. Clifton, Graham Harvey, p. 26
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Julian (emperor)97

Roman Emperor, philosopher and writer 331–363

Raheel Farooq Pakistani writer

Why I Am a Muslim: And a Christian and a Jew (2020)

“There is no crime that the Catholic Church did not commit,—no cruelty that it did not practice,—no form of treachery that it did not reward, and no virtue that it did not persecute. It was the greatest and most powerful enemy of human rights. It did all that organization, cunning, piety, self-denial, heroism, treachery, zeal and brute force could do to enslave the children of men. It was the enemy of intelligence, the assassin of liberty, and the destroyer of progress.”

Robert G. Ingersoll (1833–1899) Union United States Army officer

Rome, or Reason? A Reply to Cardinal Manning. Part I. The North American Review (1888)
Context: The people became convinced—being ignorant, stupid and credulous—that the church held the keys of heaven and hell. The foundation for the most terrible mental tyranny that has existed among men was in this way laid. The Catholic Church enslaved to the extent of its power. It resorted to every possible form of fraud; it perverted every good instinct of the human heart; it rewarded every vice; it resorted to every artifice that ingenuity could devise, to reach the highest round of power. It tortured the accused to make them confess; it tortured witnesses to compel the commission of perjury; it tortured children for the purpose of making them convict their parents; it compelled men to establish their own innocence; it imprisoned without limit; it had the malicious patience to wait; it left the accused without trial, and left them in dungeons until released by death. There is no crime that the Catholic Church did not commit,—no cruelty that it did not practice,—no form of treachery that it did not reward, and no virtue that it did not persecute. It was the greatest and most powerful enemy of human rights. It did all that organization, cunning, piety, self-denial, heroism, treachery, zeal and brute force could do to enslave the children of men. It was the enemy of intelligence, the assassin of liberty, and the destroyer of progress.

“Piety requires us to honor truth above our friends.”

Aristotle book Nicomachean Ethics

Book I, 1096a.16
Nicomachean Ethics

“Zeal ever follows an appearance of truth, and the assured are too apt to be warm; but it is their weak side in argument; zeal being better shown against sin than persons, or their mistakes.”

William Penn (1644–1718) English real estate entrepreneur, philosopher, early Quaker and founder of the Province of Pennsylvania

143
Fruits of Solitude (1682), Part I

“Ah, Postumus! they fleet away,
Our years, nor piety one hour
Can win from wrinkles and decay,
And Death's indomitable power.”
Eheu fugaces, Postume, Postume, labuntur anni nec pietas moram rugis et instanti senectae adferet indomitaeque morti.

Horace book Odes

Book II, ode xiv, line 1 (trans. John Conington)
Odes (c. 23 BC and 13 BC)

“Religious doctrines … are all illusions, they do not admit of proof, and no one can be compelled to consider them as true or to believe in them.”

Sigmund Freud book The Future of an Illusion

Source: 1920s, The Future of an Illusion (1927), Ch. 6

“Zeal will do more than knowledge.”

William Hazlitt (1778–1830) English writer

" On the Difference Between Writing and Speaking http://www.blupete.com/Literature/Essays/Hazlitt/DiffWritSpeak.htm" <br class="br">The Plain Speaker (1826)