Mark Twain [Samuel Langhorne Clemens] (1835-1910) was the writer of such classics as Tom Sawyer and Huckleberry Finn. as well as the strongly anti-religious Letters from the Earth. He was known for his biting wit and sarcasm.
The Bible is full of interest. It has noble poetry in it; and some clever fables; and some blood-drenched history; and some good morals; and a wealth of obscenity; and upwards of a thousand lies.... Our Bible reveals to us the character of our god with minute and remorseless exactness... It is perhaps the most damnatory biography that exists in print anywhere. It makes Nero an angel of light and leading by contrast...
Faith is believing something you know ain't true... A man is accepted into a church for what he believes and he is turned out for what he knows... I cannot see how a man of any large degree of humorous perception can ever be religious -- unless he purposely shut the eyes of his mind and keep them shut by force.... If Christ were here now there is one thing he would not be -- a Christian... If there is a God, he is a malign thug. -Samuel Clemens "Mark Twain", American author and humorist -- Following the Equator, Ch. 12
"Man is a marvelous curiosity . . . he thinks he is the Creator's pet . . . he even believes the Creator loves him; has a passion for him; sits up nights to admire him; yes and watch over him and keep him out of trouble. He prays to him and thinks He listens. Isn't it a quaint idea." Letters from the Earth
"Jesus died to save men -- a small thing for an immortal to do, & didn't save many, anyway; but if he had been damned for the race that would have been act of a size proper to a god, & would have saved the whole race. However, why should anybody want to save the human race, or damn it either? Does God want its society? Does Satan?" - Mark Twain, Notebook
"A man is accepted into a church for what he believes and he is turned out for what he knows." - Mark Twain 
"Most people are bothered by those passages of Scripture they do not understand, but the passages that bother me are those I do understand. - Mark Twain 
"A God who could make good children as easily a bad, yet preferred to make bad ones; who could have made every one of them happy, yet never made a single happy one; who made them prize their bitter life, yet stingily cut it short; who gave his angels eternal happiness unearned, yet required his other children to earn it; who gave is angels painless lives, yet cursed his other children with biting miseries and maladies of mind and body; who mouths justice, and invented hell -- mouths mercy, and invented hell -- mouths Golden Rules and foregiveness multiplied by seventy times seven, and invented hell; who mouths morals to other people, and has none himself; who frowns upon crimes, yet commits them all; who created man without invitation, then tries to shuffle the responsibility for man's acts upon man, instead of honorably placing it where it belongs, upon himself; and finally, with altogether divine obtuseness, invites his poor abused slave to worship him!" - Mark Twain, The Mysterious Stranger
If Christ were here there is one thing he would not be -- a Christian. - Mark Twain, Notebook
There has been only one Christian. They caught him and crucified him -- early. - Mark Twain, Notebook
"The so-called Christian nations are the most enlightened and progressive ... but in spite of their religion, not because of it. The Church has opposed every innovation and discovery from the day of Galileo down to our own time, when the use of anesthetic in childbirth was regarded as a sin because it avoided the biblical curse pronounced against Eve. And every step in astronomy and geology ever taken has been opposed by bigotry and superstition. The Greeks surpassed us in artistic culture and in architecture five hundred years before Christian religion was born." - Mark Twain, from Albert Bigelow Paine, Mark Twain, a Biography (1912), quoted from Barbara Schmidt, ed, "Mark Twain Quotations, Newspaper Collections, & Related Resources"
According to Chapter 295 of Albert Bigelow Paine's biography of him, he expressed deism on several occasions:
Let us now consider the real God, the genuine God, the great God, the sublime and supreme God, the authentic Creator of the real universe, whose remotenesses are visited by comets only comets unto which incredible distant Neptune is merely an out post, a Sandy Hook to homeward-bound specters of the deeps of space that have not glimpsed it before for generations a universe not made with hands and suited to an astronomical nursery, but spread abroad through the illimitable reaches of space by the flat of the real God just mentioned, by comparison with whom the gods whose myriads infest the feeble imaginations of men are as a swarm of gnats scattered and lost in the infinitudes of the empty sky.
At an earlier period-the date is not exactly fixable, but the stationery used and the handwriting suggest the early eighties he set down a few concisely written pages of conclusions, conclusions from which he did not deviate materially in after years. The document follows:
I believe in God the Almighty.
I do not believe He has ever sent a message to man by anybody, or delivered one to him by word of mouth, or made Himself visible to mortal eyes at any time in any place.
I believe that the Old and New Testaments were imagined and written by man, and that no line in them was authorized by God, much less inspired by Him.
I think the goodness, the justice, and the mercy of God are manifested in His works: I perceive that they are manifested toward me in this life; the logical conclusion is that they will be manifested toward me in the life to come, if there should be one.
This is supported by Mark Twain's Private War with the Almighty, which examined how his views of religion evolved over his life. After seeing how Thomas Paine got turned from hero to villain on account of his Age of Reason, he resolved to keep quiet about his religious skepticism and publicly attack only relatively "safe" targets. As the years went by and he became successful, he became relatively conciliatory toward religion, but as his fortunes reversed late in life, he became bitterly sarcastic and hostile.
External Link: Twain quotes from Positive Atheism.