“I neither oblige the belief of other person, nor overhastily subscribe mine own.”

Source: Aphorisms and Reflections (1901), p. 180

In A Man Without a Country (2005) p. 80–81 Vonnegut makes a very similar statement:
God Bless You, Dr. Kevorkian (1999)
Context: About belief or lack of belief in an afterlife: Some of you may know that I am neither Christian nor Jewish nor Buddhist, nor a conventionally religious person of any sort.
I am a humanist, which means, in part, that I have tried to behave decently without any expectation of rewards or punishments after I'm dead. My German-American ancestors, the earliest of whom settled in our Middle West about the time of our Civil War, called themselves "Freethinkers," which is the same sort of thing. My great grandfather Clemens Vonnegut wrote, for example, "If what Jesus said was good, what can it matter whether he was God or not?"
I myself have written, "If it weren't for the message of mercy and pity in Jesus' Sermon on the Mount, I wouldn't want to be a human being. I would just as soon be a rattlesnake."

Part VII, Chapter 2: On Killing
Mahayana, Śūraṅgama Sūtra

Source: The Sacred Depths of Nature (1998), p. 167
Context: We are, each one of us, ordained to live out our lives in the context of ultimate questions, such as:
Why is there anything at all, rather than nothing?
Where did the laws of physics come from?
Why does the universe seem so strange?
My response to such questions has been to articulate a covenant with Mystery. Others, of course, prefer to respond with answers, answers that often include a concept of god. These answers are by definition beliefs since they can neither be proven nor refuted. They may be gleaned from existing faith traditions or from personal search. God may be apprehended as a remote Author without present-day agency, or as an interested Presence with whom one can form a relationship, or as pantheistic — Inherent in All Things.
The opportunity to develop personal beliefs in response to questions of ultimacy, including the active decision to hold no Beliefs at all, is central to the human experience. The important part, I believe, is that the questions be openly encountered. To take the universe on — to ask Why Are Things As They Are? — is to generate the foundation for everything else.

1840s, Essays: First Series (1841), Compensation
Context: We feel defrauded of the retribution due to evil acts, because the criminal adheres to his vice and contumacy, and does not come to a crisis or judgment anywhere in visible nature. There is no stunning confutation of his nonsense before men and angels. Has he therefore outwitted the law? Inasmuch as he carries the malignity and the lie with him, he so far deceases from nature. In some manner there will be a demonstration of the wrong to the understanding also; but should we not see it, this deadly deduction makes square the eternal account.
Neither can it be said, on the other hand, that the gain of rectitude must be bought by any loss. There is no penalty to virtue; no penalty to wisdom; they are proper additions of being. In a virtuous action, I properly am; in a virtuous act, I add to the world; I plant into deserts conquered from Chaos and Nothing, and see the darkness receding on the limits of the horizon. There can be no excess to love; none to knowledge; none to beauty, when these attributes are considered in the purest sense. The soul refuses limits, and always affirms an Optimism, never a Pessimism.
His life is a progress, and not a station. His instinct is trust. Our instinct uses "more" and "less" in application to man, of the presence of the soul, and not of its absence; the brave man is greater than the coward; the true, the benevolent, the wise, is more a man, and not less, than the fool and knave. There is no tax on the good of virtue; for that is the incoming of God himself, or absolute existence, without any comparative. Material good has its tax, and if it came without desert or sweat, has no root in me, and the next wind will blow it away. But all the good of nature is the soul's, and may be had, if paid for in nature's lawful coin, that is, by labor which the heart and the head allow. I no longer wish to meet a good I do not earn, for example, to find a pot of buried gold, knowing that it brings with it new burdens. I do not wish more external goods, — neither possessions, nor honors, nor powers, nor persons. The gain is apparent; the tax is certain. But there is no tax on the knowledge that the compensation exists, and that it is not desirable to dig up treasure. Herein I rejoice with a serene eternal peace. I contract the boundaries of possible mischief. I learn the wisdom of St. Bernard, — "Nothing can work me damage except myself; the harm that I sustain I carry about with me, and never am a real sufferer but by my own fault."

Source: Echoes from the Bottomless Well (1985), p. 26

Source: Ironside

Die fast unlösbare Aufgabe besteht darin, weder von der Macht der anderen, noch von der eigenen Ohnmacht sich dumm machen zu lassen.
E. Jephcott, trans. (1974), § 34
Minima Moralia (1951)

On Becoming a Person (1961)
Source: page 23-24