“Those who wish, in the interest of morality, to reduce Leonardo, that inexhaustible source of creative power, to a neutral or sexless agency, have a strange idea of doing service to his reputation.”

2010s, 2015, Speech on (20 July 2015)

Henry Powell Spring in 1944; popularized by John F. Kennedy misquoting Dante (24 June 1963) http://www.bartleby.com/73/1211.html. Dante placed those who "non furon ribelli né fur fedeli" [were neither for nor against God] in a special region near the mouth of Hell; the lowest part of Hell, a lake of ice, was for traitors.
According to the John F. Kennedy Presidential Library & Museum http://www.jfklibrary.org/Research/Research-Aids/Ready-Reference/JFK-Quotations.aspx President Kennedy got his facts wrong. Dante never made this statement. The closest to what President Kennedy meant is in the Inferno where the souls in the ante-room of hell, who "lived without disgrace and without praise," and the coward angels, who did not rebel but did not resist the cohorts of Lucifer, are condemned to continually chase a banner that is forever changing course while being stung by wasps and horseflies.
See Canticle I (Inferno), Canto 3, vv 35-42 for the notion of neutrality and where JFK might have paraphrased from.
Misattributed

Upon The Mother Of The Gods (c. 362-363)
Context: Who then is the Mother of the Gods? She is the Source of the Intelligible and Creative Powers, which direct the visible ones; she that gave birth to and copulated with the mighty Jupiter: she that exists as a great goddess next to the Great One, and in union with the Great Creator; she that is dispenser of all life; cause of all birth; most easily accomplishing all that is made; generating without passion; creating all that exists in concert with the Father; herself a virgin, without mother, sharing the throne of Jupiter, the mother in very truth of all the gods; for by receiving within herself the causes of all the intelligible deities that be above the world, she became the source to things the objects of intellect.

At the signing of a charter establishing the German Peace Corps, Bonn, West Germany (24 June 1963);


Source: http://www.jfklibrary.org/Research/Research-Aids/Ready-Reference/JFK-Quotations.aspx According to the John F. Kennedy Presidential Library & Museum] President Kennedy got his facts wrong. Dante never made this statement. The closest to what President Kennedy meant is in the Inferno where the souls in the ante-room of hell, who "lived without disgrace and without praise," and the coward angels, who did not rebel but did not resist the cohorts of Lucifer, are condemned to being whirled through the air by great winds while being stung by wasps and horseflies. Dante placed those who "non furon ribelli né fur fedeli" — were neither for nor against God, in a special region near the mouth of Hell; the lowest part of Hell, a lake of ice, was for traitors.
Source: https://web.archive.org/web/20201213100425/https://www.jfklibrary.org/learn/about-jfk/life-of-john-f-kennedy/fast-facts-john-f-kennedy/john-f-kennedys-favorite-quotations-dantes-inferno According to the John F. Kennedy Presidential Library & Museum in the undated article "John F. Kennedy's Favorite Quotations: Dante's Inferno"
President Kennedy's quote was based upon an interpretation of Dante's Inferno. As Robert Kennedy explained in 1964, "President Kennedy's favorite quote was really from Dante, 'The hottest places in Hell are reserved for those who in time of moral crisis preserve their neutrality.'" This supposed quotation is not actually in Dante's work, but is based upon a similar one. In the Inferno, Dante and his guide Virgil, on their way to Hell, pass by a group of dead souls outside the entrance to Hell. These individuals, when alive, remained neutral at a time of great moral decision. Virgil explains to Dante that these souls cannot enter either Heaven or Hell because they did not choose one side or another. They are therefore worse than the greatest sinners in Hell because they are repugnant to both God and Satan alike, and have been left to mourn their fate as insignificant beings neither hailed nor cursed in life or death, endlessly travailing below Heaven but outside of Hell. This scene occurs in the third canto of the Inferno.
Source: http://www.bartleby.com/73/1211.html 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.”

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.

TV Interview for Thames TV TV Eye (24 April 1979) http://www.margaretthatcher.org/document/104040
Leader of the Opposition

Source: Academy Series - Priscilla "Hutch" Hutchins, Deepsix (2001), Chapter 11 (p. 180)