“But I suppose the most revolutionary act one can engage in is… to tell the truth.”

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.

The first number of L'Ordine Nuovo, edited by Gramsci, appeared in 1921 with this motto of Ferdinand Lassalle on the first page. It is often misattributed to Gramsci.
Misattributed

Ibid., p. 6
The Education of the Stoic

Source: The Rosa Luxemburg Reader

As quoted in Commentary: The gaffer speaks, The Times, April 23, 1988.

"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)

Concepts