“All religion seems to need to prove that it's the only truth.”

The Artful Universe (1995)
Context: If a 'religion' is defined to be a system of ideas that contains unprovable statements, then Gödel taught us that mathematics is not only a religion, it is the only religion that can prove itself to be one.<!-- Ch. 5, p. 211

Source: Honest Illusions

The Uttarpara Address (1909)
Context: That which we call the Hindu religion is really the eternal religion, because it is the universal religion which embraces all others. If a religion is not universal, it cannot be eternal. A narrow religion, a sectarian religion, an exclusive religion can live only for a limited time and a limited purpose. This is the one religion that can triumph over materialism by including and anticipating the discoveries of science and the speculations of philosophy. It is the one religion which impresses on mankind the closeness of God to us and embraces in its compass all the possible means by which man can approach God. It is the one religion which insists every moment on the truth which all religions acknowledge that He is in all men and all things and that in Him we move and have our being. It is the one religion which enables us not only to understand and believe this truth but to realise it with every part of our being. It is the one religion which shows the world what the world is, that it is the Lila of Vasudeva. It is the one religion which shows us how we can best play our part in that Lila, its subtlest laws and its noblest rules. It is the one religion which does not separate life in any smallest detail from religion, which knows what immortality is and has utterly removed from us the reality of death.

Fourth Realm Trilogy (2005-2009), The Traveler (2005)

Variants (these could be paraphrases or differing translations): The belief that there is only one truth and that oneself is in possession of it seems to me the deepest root of all evil that is in the world.
The belief that there is only one truth, and that oneself is in possession of it, is the root of all evil in the world.
Source: Natural Philosophy of Cause and Chance (1964), p. 230, also in My Life and Views (1968), p. 183

Source: What is Property? (1840), Ch. I: "Method Pursued in this Work. The Idea of a Revolution"
Context: To name a thing is easy: the difficulty is to discern it before its appearance. In giving expression to the last stage of an idea, — an idea which permeates all minds, which to-morrow will be proclaimed by another if I fail to announce it to-day, — I can claim no merit save that of priority of utterance. Do we eulogize the man who first perceives the dawn?
Yes: all men believe and repeat that equality of conditions is identical with equality of rights; that property and robbery are synonymous terms; that every social advantage accorded, or rather usurped, in the name of superior talent or service, is iniquity and extortion. All men in their hearts, I say, bear witness to these truths; they need only to be made to understand it.

Lectures on Philosophy of Religion, Volume 1 (1827)

The Bloudy Tenent of Persecution, for Cause of Conscience (1644)

Phrases and Philosophies for the Use of the Young (1894)