Quoting from Mark Twain's book "Christian Science"

But we all know that conversions are seldom made in that way; that such a thing as a serious and painstaking and fairly competent inquiry into the claims of a religion or of a political dogma is a rare occurrence; and that the vast mass of men and women are far from being capable of making such an examination. They are not capable, for the reason, that their minds, howsoever good they may be, are not trained for such examinations.

the only people allowed to be social critics are humorists. that way their words don't have to be taken seriously.

"…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 forgiveness 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

"During many ages there were witches. The Bible said so. The Bible commanded that they should not be allowed to live. Therefore the Church, after eight hundred years, gathered up its halters, thumb-screws, and firebrands, and set about its holy work in earnest. She worked hard at it night and day during nine centuries and imprisoned, tortured, hanged, and burned whole hordes and armies of witches, and washed the Christian world clean with their foul blood. Then it was discovered that there was no such thing as witches, and never had been. One does not know whether to laugh or to cry… There are no witches. The witch text remains; only the practice has changed."
-- Mark Twain, Europe and Elsewhere, "Bible Teaching and Religious Practice"

"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."
-- Mark Twain, a Biography

"If I were to construct a God I would furnish Him with some way and qualities and characteristics which the Present lacks. He would not stoop to ask for any man's compliments, praises, flatteries; and He would be far above exacting them. I would have Him as self-respecting as the better sort of man in these regards. He would not be a merchant, a trader. He would not buy these things. He would not sell, or offer to sell, temporary benefits of the joys of eternity for the product called worship. I would have Him as dignified as the better sort of man in this regard. He would value no love but the love born of kindnesses conferred; not that born of benevolences contracted for. Repentance in a man's heart for a wrong done would cancel and annul that sin; and no verbal prayers for forgiveness be required or desired or expected of that man. In His Bible there would be no Unforgiveable Sin. He would recognize in Himself the Author and Inventor of Sin and Author and Inventor of the Vehicle and Appliances for its commission; and would place the whole responsibility where it would of right belong: upon Himself, the only Sinner. He would not be a jealous God -- a trait so small that even men despise it in each other. He would not boast. He would keep private Hs admirations of Himself; He would regard self-praise as unbecoming the dignity of his position. He would not have the spirit of vengeance in His heart. Then it would not issue from His lips. There would not be any hell -- except the one we live in from the cradle to the grave. There would not be any heaven -- the kind described in the world's Bibles. He would spend some of His eternities in trying to forgive Himself for making man unhappy when he could have made him happy with the same effort and he would spend the rest of them in studying astronomy."
-- Mark Twain's Notebook

"The best minds will tell you that when a man has begotten a child he is morally bound to tenderly care for it, protect it from hurt, shield it from disease, clothe it, feed it, bear with its waywardness, lay no hand upon it save in kindness and for its own good, and never in any case inflict upon it a wanton cruelty. God's treatment of his earthly children, every day and every night, is the exact opposite of all that, yet those best minds warmly justify these crimes, condone them, excuse them, and indignantly refuse to regard them as crimes at all, when he commits them. Your country and mine is an interesting one, but there is nothing there that is half so interesting as the human mind."
-- Mark Twain, Letters from the Earth

"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… The higher animals have no religion. And we are told that they are going to be left out in the Hereafter. I wonder why? It seems questionable taste."
-- Mark Twain, The Lowest Animal essay, 1897

"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. 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, a Biography