Emerson left the ministry over a sacrament. In 1832, three years into his pastorate at the Second Church in Boston, he told his congregation that he no longer believed the Lord's Supper had been instituted as a permanent rite and that he could not administer it sincerely. They declined to dispense with it. He resigned, travelled in Europe, met Coleridge, Wordsworth and Carlyle, and came home without a pulpit and without a settled occupation.
Nature, published anonymously in 1836, is what came out of it. Its argument is that the natural world is not raw material and not decoration but the visible face of spirit, legible to anyone who looks at it directly, and that every generation is entitled to its own encounter rather than to a report of somebody else's. The opening question, why should we not have an original relation to the universe, is the whole programme in one sentence.
Underneath sits the idea he called the Over-Soul: a single mind or ground in which every individual mind participates, so that what is deepest in a person is not private property but universal. That is why self reliance in his usage is not egotism. Trusting yourself, on this account, is trusting the universal that speaks through you, and the injunction to do so is closer to a religious duty than to advice about confidence.