“For he who reckons it a pleasure that a man, though justly condemned, should be slain in his sight, pollutes his conscience as much as if he should become a spectator and a sharer of a homicide which is secretly committed.”
Book VI, Chap. XX
The Divine Institutes (c. 303–13)
Original
Nam qui hominem, quamuis ob merita damnatum, in conspectu suo iugulari pro uoluptate computat, conscientiam suam polluit, tam scilicet, quam si homicidii, quod fit occulte, spectator et particeps fiat.
Help us to complete the source, original and additional information
Lactantius 3
Early Christian author 250–325Related quotes

"The Crime against Kansas," speech in the Senate (May 18, 1856). The claims made against Senator Andrew Butler of South Carolina so angered Butler's cousin, Democrat Representative Preston Brooks, that Brooks assaulted Sumner with a cane in the Senate chamber a few weeks later

The Ethics of Belief (1877), The Duty of Inquiry
Context: The harm which is done by credulity in a man is not confined to the fostering of a credulous character in others, and consequent support of false beliefs. Habitual want of care about what I believe leads to habitual want of care in others about the truth of what is told to me. Men speak the truth of one another when each reveres the truth in his own mind and in the other's mind; but how shall my friend revere the truth in my mind when I myself am careless about it, when I believe thing because I want to believe them, and because they are comforting and pleasant? Will he not learn to cry, "Peace," to me, when there is no peace? By such a course I shall surround myself with a thick atmosphere of falsehood and fraud, and in that I must live. It may matter little to me, in my cloud-castle of sweet illusions and darling lies; but it matters much to Man that I have made my neighbours ready to deceive. The credulous man is father to the liar and the cheat; he lives in the bosom of this his family, and it is no marvel if he should become even as they are.

Shulchan Arukh, Yoreh De'a, 246:21, as cited in "Separation from the Worldly (Perishut)" http://etzion.org.il/en/separation-worldly-perishut

“A fanatic is a man who, when he's lost sight of his purpose, redoubles his effort.”
Source: Harvest of Stars

You Can Lead an Atheist to Evidence, But You Can't Make Him Think (2009)