A few favorites:
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.
- Autobiographical dictation, 10 July 1908. Published in Autobiography of Mark Twain, Volume 3 (University of California Press, 2015)
The easy confidence with which I know another man's religion is folly teaches me to suspect that my own is also. I would not interfere with any one's religion, either to strengthen it or to weaken it. I am not able to believe one's religion can affect his hereafter one way or the other, no matter what that religion may be. But it may easily be a great comfort to him in this life--hence it is a valuable possession to him.
- Mark Twain, a Biography