Adler was the son of the rabbi of Temple Emanu-El in New York and was expected to succeed him. He studied in Germany, concluded he could not honestly affirm a personal God, and in 1876 founded the New York Society for Ethical Culture instead.
The organising principle was deed before creed: that the ethical life is what the religions have in common and what actually matters, that it stands on its own without theological support, and that a congregation can therefore be built without requiring members either to affirm or to deny anything about God. This was not designed as a compromise. Adler held that moral obligation arises from the worth of other persons rather than from divine command, and that grounding ethics in theology in fact makes it less secure.
The interesting legal consequence is that Ethical Culture is recognised as a religion in United States law while holding no doctrine about any deity, established in the courts in the 1950s for tax and conscientious-objection purposes, and cited by the Supreme Court in Torcaso v. Watkins (1961) among religions that do not teach belief in a god. Whichever way one reads it, the case demonstrates that the legal category of religion in America turns on function and sincerity rather than on theism.
He put the principle to work rather than into doctrine: a free kindergarten that became the Ethical Culture Fieldston School, district nursing, tenement reform, and the chairmanship of the National Child Labor Committee during the campaign against child labour.