“Fire destroys all sophistry, that is deceit; and maintains truth alone, that is gold.”

The Notebooks of Leonardo da Vinci (1883), X Studies and Sketches for Pictures and Decorations

or subtle things
The Notebooks of Leonardo da Vinci (1883), X Studies and Sketches for Pictures and Decorations

Aeschines, In Timarchum, 84 (107).

“A Deceit,” p. 29
The Sun Watches the Sun (1999), Sequence: “Skywalking”

City of Lost Souls

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: Social Amnesia: A Critique of Conformist Psychology from Adler to Laing (1975), p. 23-25
Context: The Adlerians, in the name of “individual psychology,” take the side of society against the individual. … Adler’s later thought succumbs to the worst of his earlier banalization. It is conventional, practical, and moralistic. “Our science … is based on common sense.” Common sense, the half-truths of a deceitful society, is honored as the honest truths of a frank world.

De Providentia (On Providence): cap. 5, line 9
Alternate translation: Fire is the test of gold; adversity, of strong men. (translator unknown).
Moral Essays

No. 2, The Look of Love
1790s, Poems from Blake's Notebook (c. 1791-1792), Several Questions Answered