“When we see religion split into so many thousand of sects, and I may say Christianity itself divided into its thousands also, who are disputing, anathematizing and where the laws permit burning and torturing one another for abstractions which no one of them understand, and which are indeed beyond the comprehension of the human mind, into which of the chambers of this Bedlam would a man wish to thrust himself.[Letter to George Logan, 12 November 1816]”

“I write nothing for publication, and last of all things should it be on the subject of religion. On the dogmas of religion as distinguished from moral principles, all mankind, from the beginning of the world to this day, have been quarrelling, fighting, burning and torturing one another, for abstractions unintelligible to themselves and to all others, and absolutely beyond the comprehension of the human mind. Were I to enter on that arena, I should only add an unit to the number of Bedlamites.[Letter to Mathew Carey, 11 November 1816]”

“Faith belongs to the human spirit. Faith is faith. Humanity is divided by religion, religion is the divider of humanity. If every human could be removed of their blindfolds and see that faith is in itself faith and that this is something which belongs to each and every human being, then at that time the dividers of religion will suddenly mean nothing and we will all see that we are united by faith in and of itself. There is only one faith and it is called faith. And no man needs to prove to another man that what he believes in exists, because even if it does not exist, his faith is his belief that it is there, that something is there, and that in itself is faith. So I do not need to prove to any man that what I believe in exists or not, there is no such contest between man, my faith breathes in the body of my belief; the fact that I believe is the breath of my faith.”

“The hocus-pocus phantasm of a God like another Cerberus, with one body and three heads, had its birth and growth in the blood of thousands and thousands of martyrs... In fact, the Athanasian paradox that one is three, and three but one, is so incomprehensible to the human mind, that no candid man can say he has any idea of it, and how can he believe what presents no idea? He who thinks he does, only deceives himself. He proves, also, that man, once surrendering his reason, has no remaining guard against absurdities the most monstrous, and like a ship without a rudder, is the sport of every wind. With such persons, gullibility which they call faith, takes the helm from the hand of reason, and the mind becomes a wreck.[Letter to James Smith discussing Jefferson's hate of the doctrine of the Christian trinity, December 8 1822]”

“A man cannot realize that above such shattered bodies there are still human faces in which life goes its daily round. And this is only one hospital, a single station; there are hundreds of thousands in Germany, hundreds of thousands in France, hundreds of thousands in Russia. How senseless is everything that can ever be written, done, or thought, when such things are possible. It must be all lies and of no account when the culture of a thousand years could not prevent this stream of blood being poured out, these torture chambers in their hundreds of thousands. A hospital alone shows what war is.”

“The range of the human mind, the scale and depth of the metaphors the mind is capable of manufacturing as it grapples with the universe, stand in stunning contrast to the belief that there is only one reality, which is man's, or worse, that only one culture among the many on earth possesses the truth.To allow mystery, which is to say to yourself, "There could be more, there could be things we don't understand," is not to damn knowledge. It is to take a wider view. It is to permit yourself an extraordinary freedom: someone else does not have to be wrong in order that you may be right.”