“To tell the truth is revolutionary.”

No source for this quote among Orwell's writings has yet been located, and the earliest published source of this phrase found on Google Books is this snippet https://books.google.com.mx/books?id=kWD0AAAAMAAJ&q=%22truth+is+a+revolutionary+act%22&dq=%22truth+is+a+revolutionary+act%22&hl=en&sa=X&ved=0ahUKEwjs0MKSqpbKAhWH0iYKHXj6ABUQ6AEIJjAD from p. 5 of Science Dimension, Volumes 14–18 (1982) published by the National Research Council Canada. Quote Investigator has an article "In a Time of Universal Deceit – Telling the Truth Is a Revolutionary Act" http://quoteinvestigator.com/2013/02/24/truth-revolutionary/ indicating their attempts to trace the quote. The earliest similar remarks they had found were in a 1982 book titled “Partners in Ecocide: Australia’s Complicity in the Uranium Cartel” by Venturino Giorgio Venturini, where the word “universal” was omitted, and a specific originating text was not identified: "In a time of deceit, telling the truth is a revolutionary act."
Variants:
During times of universal deceit, telling the truth becomes a revolutionary act.
In an age of universal deceit, telling the truth is a revolutionary act.
In a time of universal deceit, telling the truth is a revolutionary act.
Speaking the Truth in times of universal deceit is a revolutionary act.
Truth is treason in an empire of lies. (Often attributed by Ron Paul to Orwell but never sourced.)
In the mid-19th century Karl Georg von Raumer made a remark, which has a similar meaning. In Geschichte der Pedagogic (1855), he states: 'Jede keimende Wahrheit ist revolutionär gegen den entgegenstehenden herrschenden Irrthum, jede keimende Tugend revolutionär gegen das im Schwange gehende, ihr widersprechende Laster' which translates as: "Every germinating truth is revolutionary against the opposing ruling error, every germinating virtue is revolutionary against popular contradictory lies."
In 1898 French socialist Jean Jaurès said, "When a society, when an institution, lives only by lies, truth is revolutionary." He was speaking with reference to the ongoing Dreyfus Affair. The statement is quoted in Ruth Harris, The Man on Devil's Island: Alfred Dreyfus and the Affair that Divided France (2010), p. 262. (She cites Le petit Meridional, 3 July 1898, as the original source.) This seems very close in spirit and in phrasing to the pseudo-Orwell quotation. (The cumulative index to the many volumes of Orwell's writing compiled and edited by Peter Davison does not reveal any direct references to Jaurès or the Dreyfus Affair.)
Disputed
Variant: In a time of universal deceit, telling the truth is a revolutionary act.

Source: Marx in Soho: A Play on History

Camp Columbia, Havana (Jan. 8th, 1959), Fidel Castro Reader, pp. 133

Source: Contributions to the history and improvement of the german universities - A history of pedagogy; volume 4 (1855), p. 99

Preface to the Third Edition (August 1942)
The Mass Psychology of Fascism (1933)
Context: If, by being revolutionary, one means rational rebellion against intolerable social conditions, if, by being radical, one means "going to the root of things," the rational will to improve them, then fascism is never revolutionary. True, it may have the aspect of revolutionary emotions. But one would not call that physician revolutionary who proceeds against a disease with violent cursing but the other who quietly, courageously and conscientiously studies and fights the causes of the disease. Fascist rebelliousness always occurs where fear of the truth turns a revolutionary emotion into illusions.

1129: Tell all the Truth but tell it slant —
The Complete Poems of Emily Dickinson (1960)
Variant: Tell all the Truth but tell it slant —
Success in Circuit lies
Source: The Complete Poems of Emily Dickinson
Context: p>Tell all the Truth but tell it slant —
Success in Circuit lies
Too bright for our infirm Delight
The Truth's superb surpriseAs Lightning to the Children eased
With explanation kind
The Truth must dazzle gradually
Or every man be blind —</p

The Visitor (2002)
Context: The Regime has become so smug it can't tell the difference among the revolutionary, the innovative, or the merely various. The high command knows so little about the outside that if I came back with a fully equipped chemical laboratory and told them I'd found it in a cave, they'd probably believe that, so long as I brought it back piecemeal in my saddle bags, thus proving I hadn't known it was there beforehand.

Ch. 37 : leaving bastion

"Life After Lebanon" (1984), later published in On Call : Political Essays (1985), and Some of Us Did Not Die : New and Selected Essays of June Jordan (2002)

Alan Dershowitz, The Best Defense (New York: Vintage), 1983-5-12, p. xiv.