I chose this quote because it was the shortest. It's from the "Autobiography of Mark Twain, Vol. 2":
"We brazenly call our God the source of mercy, while we are aware, all the time, that there is not an authentic instance in history of His ever having exercised that virtue. We call Him the source of morals, while we know by His history and by His daily conduct, as perceived with our own senses, that He is totally destitute of anything resembling morals. We call Him Father, and not in derision, although we would detest and denounce any earthly father who should inflict upon his child a thousandth part of the pains and miseries and cruelties which our God deals out to His children every day, and has dealt out to them daily during all the centuries since the crime of creating Adam was committed."
It's a good thing Twain didn't mention God's followers or else, considering the times he lived in and the sort of man he was, I'd have had to probably pick something longer,...
"There are many humorous things in this world; among them the white man's notion that he is less savage than the other savages."