Ethical Culture was founded on a proposition that sounded eccentric in 1876 and now sounds obvious: that people who disagree entirely about God can nonetheless agree about how to behave, and that the agreement is enough to build a congregation on.
Felix Adler was the son of the rabbi of Temple Emanu-El in New York, and had been trained to succeed him. He returned from doctoral study in Germany unable to affirm a personal God, gave an address in 1876 setting out a religion of ethics rather than of belief, and the New York Society for Ethical Culture was formally constituted the following year. Both dates appear in the sources, and the accurate statement is that the address came in 1876 and the society in 1877.
The motto is deed before creed. Members are neither required to affirm nor to deny a god; what is asked is a commitment to act so as to elicit the best in others and thereby in oneself. Adler took from Kant the principle that persons are to be treated as ends and never merely as means, and made it the working centre of a congregation rather than a proposition in a seminar.
The form is congregational and looks, deliberately, like a church. There is a Sunday meeting with a half-hour address, called the Platform, and a Sunday school for children. The clergy are called Leaders, are formally trained and certified, and are called by societies much as Protestant ministers are.
Adler was not excommunicated and did not intend a break with Judaism so much as a move past the question that had stopped him. His father's congregation understood perfectly well what had happened, and he never took the pulpit he had been raised to take.