“I cheerfully admit that we are indebted to Christianity for some learning, and that the human mind has been developed by the discussion of the absurdities of superstition. Certainly millions and millions have had what might be called mental exercise, and their minds may have been somewhat broadened by the examination, even, of these absurdities, contradictions, and impossibilities.”

Source: The Voice of Destruction (1940), p. 186

Source: Disputed, Hitler Speaks (1940), p. 186

Heretics and Heresies (1874)
Context: I do not say, and I do not believe, that Christians are as bad as their creeds. In spite of church and dogma, there have been millions and millions of men and women true to the loftiest and most generous promptings of the human heart. They have been true to their convictions, and, with a self-denial and fortitude excelled by none, have labored and suffered for the salvation of men. Imbued with the spirit of self-sacrifice, believing that by personal effort they could rescue at least a few souls from the infinite shadow of hell, they have cheerfully endured every hardship and scorned every danger. And yet, notwithstanding all this, they believed that honest error was a crime. They knew that the Bible so declared, and they believed that all unbelievers would be eternally lost. They believed that religion was of God, and all heresy of the devil. They killed heretics in defence of their own souls and the souls of their children. They killed them because, according to their idea, they were the enemies of God, and because the Bible teaches that the blood of the unbeliever is a most acceptable sacrifice to heaven.

A Christmas Sermon (1890)
Context: Christian chronology gives the age of the first man, and then gives the line from father to son down to the flood, and from the flood down to the coming of Christ, showing that men have been upon the earth only about six thousand years. This chronology is infinitely absurd, and I do not believe that there is an intelligent, well-educated Christian in the world, having examined the subject, who will say that the Christian chronology is correct.

Source: Mort

Rome, or Reason? A Reply to Cardinal Manning. Part I. The North American Review (1888)
Context: "Whosoever will be saved, before all things it is necessary that he hold the Catholic faith." It is not necessary, before all things, that he be good, honest, merciful, charitable and just. Creed is more important than conduct. The most important of all things is, that he hold the Catholic faith. There were thousands of years during which it was not necessary to hold that faith, because that faith did not exist; and yet during that time the virtues were just as important as now, just as important as they ever can be. Millions of the noblest of the human race never heard of this creed. Millions of the bravest and best have heard of it, examined, and rejected it. Millions of the most infamous have believed it, and because of their belief, or notwithstanding their belief, have murdered millions of their fellows. We know that men can be, have been, and are just as wicked with it as without it.

Is Shakespeare Dead? (1909), §11, as reprinted in Essays and Sketches of Mark Twain (1995), ed. Stuart Miller, ISBN 1566198798

Source: Guide for the Perplexed (c. 1190), Part III, Ch.17

Speech to the International Eucharistic Conference, Philadelphia, Pennsylvania as quoted in the Los Angeles Herald-Examiner (13 August 1976)
1970s