“Who is, in the classical conception, the subject that comprehends the ontological condition of truth and untruth? It is the master of pure contemplation (theoria), and the master of a practice guided by theoria, i. e., the philosopher-statesman. To be sure, the truth which he knows and expounds is potentially accessible to everyone. Led by the philosopher, the slave in Plato’s Meno is capable of grasping the truth of a geometrical axiom, i. e., a truth beyond change and corruption. But since truth is a state of Being as well as of thought, and since the latter is the expression and manifestation of the former, access to truth remains mere potentiality as long as it is not living in and with the truth. And this mode of existence is closed to the slave — and to anyone who has to spend his life procuring the necessities of life. Consequently, if men no longer had to spend their lives in the realm of necessity, truth and a true human existence would be in a strict and real sense universal. Philosophy envisages the equality of man but, at the same time, it submits to the factual denial of equality. For in the given reality, procurement of the necessities is the life-long job of the majority, and the necessities have to be procured and served so that truth (which is freedom from material necessities) can be. Here, the historical barrier arrests and distorts the quest for truth; the societal division of labor obtains the dignity of an ontological condition. If truth presupposes freedom from toil, and if this freedom is, in the social reality, the prerogative of a minority, then the reality allows such a truth only in approximation and for a privileged group. This state of affairs contradicts the universal character of truth, which defines and “prescribes” not only a theoretical goal, but the best life of man qua man, with respect to the essence of man. For philosophy, the contradiction is insoluble, or else it does not appear as a contradiction because it is the structure of the slave or serf society which this philosophy does not transcend. Thus it leaves history behind, unmastered, and elevates truth safely above the historical reality. There, truth is reserved intact, not as an achievement of heaven or in heaven, but as an achievement of thought — intact because its very notion expresses the insight that those who devote their lives to earning a living are incapable of living a human existence.”

Preface to Second Edition (1843)
The Essence of Christianity (1841)

Source: Persecution and the Art of Writing (1952), Persecution and the Art of Writing, p. 35

Source: The Secret Oral Teachings in the Tibetan Buddhist Sects (1964)

Source: Practical Mysticism (1914), Chapter I, What Is Mysticism?, p. 28

[Alan, Dershowitz, http://online.wsj.com/article/SB10001424052702303544604576429783247016492.html?mod=WSJ_Opinion_LEADTop, Casey Anthony: The System Worked, The Wall Street Journal, July 7, 2011, July 7, 2011] published 2011-07-07

Source: Echoes from the Bottomless Well (1985), p. 94

Source: As You Like It

King quoted here John F. Kennedy who at the signing of a charter establishing the German Peace Corps in Bonn, West Germany (24 June 1963) remarked: Dante once said that the hottest places in hell are reserved for those who, in a period of moral crisis, maintain their neutrality.
According to Bartleby.com, Kennedy's remark may have been inspired by the passage from Dante Alighieri’s La Comedia Divina “Inferno,” canto 3, lines 35–42 (1972) passage as translated by Geoffrey L. Bickersteth: "by those disbodied wretches who were loth when living, to be either blamed or praised. [...] Fear to lose beauty caused the heavens to expel these caitiffs; nor, lest to the damned they theng ave cause to boast, receives them the deep hell." A more modern-sounding translation from the foregoing Dante’s Inferno passage was translataed 1971 by Mark Musa thus: “They are mixed with that repulsive choir of angels … undecided in neutrality. Heaven, to keep its beauty, cast them out, but even Hell itself would not receive them for fear the wicked there might glory over them.”
This is also often quoted slightly differently as: "The hottest place in Hell is reserved for those who remain neutral in times of great moral conflict"
1960s, Why I Am Opposed to the War in Vietnam (1967)
Context: I see this war as an unjust, evil, and futile war. I preach to you today on the war in Vietnam because my conscience leaves me with no other choice. The time has come for America to hear the truth about this tragic war. In international conflicts, the truth is hard to come by because most nations are deceived about themselves. Rationalizations and the incessant search for scapegoats are the psychological cataracts that blind us to our sins. But the day has passed for superficial patriotism. He who lives with untruth lives in spiritual slavery. Freedom is still the bonus we receive for knowing the truth. "Ye shall know the truth," says Jesus, "and the truth shall set you free." Now, I've chosen to preach about the war in Vietnam because I agree with Dante, that the hottest places in hell are reserved for those who in a period of moral crisis maintain their neutrality. There comes a time when silence becomes betrayal.

Source: Aphorisms and Reflections (1901), p. 273