“I believe that ignorance is the root of all evil.
And that no one knows the truth.”

Variants (these could be paraphrases or differing translations): The belief that there is only one truth and that oneself is in possession of it seems to me the deepest root of all evil that is in the world.
The belief that there is only one truth, and that oneself is in possession of it, is the root of all evil in the world.
Source: Natural Philosophy of Cause and Chance (1964), p. 230, also in My Life and Views (1968), p. 183

Suffering
Source: I am That, P.116.

As quoted in The Martians of Science : Five Physicists Who Changed the Twentieth Century (2006) by István Hargittai, p. 251
Context: I believe in evil. It is the property of all those who are certain of truth. Despair and fanaticism are only differing manifestations of evil.

"What I Believe" in The Forum 84 (September 1930), p. 139; some of these expressions were also used separately in other Mencken essays.
1930s
Context: I believe that religion, generally speaking, has been a curse to mankind — that its modest and greatly overestimated services on the ethical side have been more than overcome by the damage it has done to clear and honest thinking.
I believe that no discovery of fact, however trivial, can be wholly useless to the race, and that no trumpeting of falsehood, however virtuous in intent, can be anything but vicious.
I believe that all government is evil, in that all government must necessarily make war upon liberty and the democratic form is as bad as any of the other forms.
I believe that the evidence for immortality is no better than the evidence of witches, and deserves no more respect.
I believe in the complete freedom of thought and speech — alike for the humblest man and the mightiest, and in the utmost freedom of conduct that is consistent with living in organized society.
I believe in the capacity of man to conquer his world, and to find out what it is made of, and how it is run.
I believe in the reality of progress.
I —But the whole thing, after all, may be put very simply. I believe that it is better to tell the truth than to lie. I believe that it is better to be free than to be a slave. And I believe that it is better to know than be ignorant.

Source: Rich Dad, Poor Dad: What the Rich Teach Their Children About Money That the Poor and Middle Class Don't

Source: Either/Or: A Fragment of Life

Imperialismo Pagano (1928) · Excerpts http://www.juliusevola.net/excerpts/Tradition_of_the_Mysteries_against_Christianity.html

This appears in Twain's posthumous The Refuge of the Derelicts (1905), but it had already been published by other writers.
The earliest citation found in Google Books is a 1872 article by Richard Bowker: "Our Crime Against Crimes" https://books.google.com/books?id=YZgBAAAAYAAJ&pg=PA68&dq=The+lack+of+money+is+the+root+of+all+evil&hl=en&sa=X&ved=0ahUKEwjWi5DE1crLAhUI3mMKHeSdB0YQ6AEIKzAB#v=onepage&q=%22lack%20of%20money%22&f=false, in The Herald of Health, vol. 19 no. 2, New York: Wood & Holbrook, February 1872. The saying is placed within quotation marks, perhaps indicating that it was already well-known.
A precursor is found in an article from 1859 https://books.google.com/books?id=gpdEAQAAMAAJ&pg=PA209&dq=The+lack+of+money+is+the+root+of+all+evil&hl=en&sa=X&ved=0ahUKEwjWi5DE1crLAhUI3mMKHeSdB0YQ6AEINTAD#v=onepage&q=%22lack%20of%20gold%22&f=false: It is very well to repeat, parrot-like, the old axiom that “the love of gold is the root of all evil;” but it is very certain that in truth—the lack of gold is the great incentive to crime.
Disputed