Twain generally avoided publishing his most "heretical" opinions on religion in his lifetime, and they are known from essays and stories that were published later. In the essay Three Statements of the Eighties in the 1880s, Twain stated that he believed in an almighty God, but not in any messages, revelations, holy scriptures such as the Bible, providence, or retribution in the afterlife. He did believe that "the goodness, the justice, and the mercy of God are manifested in His works", but also that "the universe is governed by strict and immutable laws", which determine "small matters" such as who dies in a pestilence. In later writings in the 1890s, he was less optimistic about the goodness of God, observing that "if our Maker is all-powerful for good or evil, He is not in His right mind". At other times, he conjectured sardonically that perhaps God had created the world with all its tortures for some purpose of His own, but was otherwise indifferent to humanity, which was too petty and insignificant to deserve His attention anyway.
Twain had a unique ability to express profound truths in a pithy way. Here are some of his better quotes on religion.
1. The Church has opposed every innovation and discovery from the day of Galileo down to our own time, when the use of anesthetics in childbirth was regarded as a sin because it avoided the biblical curse pronounced against Eve.
2. So much blood has been shed by the Church because of an omission from the Gospel: "Ye shall be indifferent as to what your neighbor's religion is." Not merely tolerant of it, but indifferent to it. Divinity is claimed for many religions; but no religion is great enough or divine enough to add that new law to its code.
3. Man is a Religious Animal. He is the only Religious Animal. He is the only animal that has the True Religion--several of them. He is the only animal that loves his neighbor as himself and cuts his throat if his theology isn't straight. He has made a graveyard of the globe in trying his honest best to smooth his brother's path to happiness and heaven....
4. In religion and politics people's beliefs and convictions are in almost every case gotten at second-hand, and without examination, from authorities who have not themselves examined the questions at issue but have taken them at second-hand from other non-examiners, whose opinions about them were not worth a brass farthing.
5. ...a God who could make good children as easily as bad ones, 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 his 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!
6. Faith is believing something you know ain't true.
7. 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.
8. There is one notable thing about our Christianity: bad, bloody, merciless, money-grabbing and predatory as it is -- in our country particularly, and in all other Christian countries in a somewhat modified degree -- it is still a hundred times better than the Christianity of the Bible, with its prodigious crime -- the invention of Hell. Measured by our Christianity of to-day, bad as it is, hypocritical as it is, empty and hollow as it is, neither the Deity nor His Son is a Christian, nor qualified for that moderately high place. Ours is a terrible religion. The fleets of the world could swim in spacious comfort in the innocent blood it has spilt.
9. Blasphemy? No, it is not blasphemy. If God is as vast as that, he is above blasphemy; if He is as little as that, He is beneath it.
10. The War Prayer: O Lord our God, help us tear their soldiers to bloody shreds with our shells; help us to cover their smiling fields with the pale forms of their patriot dead; help us to drown the thunder of the guns with the shrieks of their wounded, writhing in pain; help us to lay waste their humble homes with a hurricane of fire; help us to wring the hearts of their unoffending widows with unavailing grief; help us to turn them out roofless with their little children to wander unfriended the wastes of their desolated land in rags and hunger and thirst, sports of the sun flames of summer and the icy winds of winter, broken in spirit, worn with travail, imploring Thee for the refuge of the grave and denied it...
11. [The Bible is] a mass of fables and traditions, mere mythology.
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.
13. The church is always trying to get other people to reform; it might not be a bad idea to reform itself a little, by way of example.
14. If Christ were here now there is one thing he would not be -- a Christian.
15. There are many scapegoats for our sins, but the most popular one is Providence.
16. One of the proofs of the immortality of the soul is that myriads have believed it. They also believed the world was flat.
17. Go to Heaven for the climate, Hell for the company.
18. Sacred cows make the best hamburger.
19. Religion consists in a set of things which the average man thinks he believes and wishes he was certain of.
20. There is no other life; life itself is only a vision and a dream for nothing exists but space and you. If there was an all-powerful God, he would have made all good, and no bad.
21. "In God We Trust." I don't believe it would sound any better if it were true.
22. If there is a God, he is a malign thug.
23. [The Bible] has noble poetry in it... and some good morals and a wealth of obscenity, and upwards of a thousand lies.
And my all time favorite quote:
24. It ain't the parts of the Bible that I can't understand that bother me, it is the parts that I do understand.
For more on Twain's views on religion, see The Bible According to Mark Twain: Irreverent Writings on Eden, Heaven, and the Flood by America's Master Satirist