The phrase most closely associated with Creed and Deed never actually appears in that exact wording inside the book, but the argument it summarizes runs through every discourse Adler collected there. His claim was blunt: a person's actual conduct toward other people is the true measure of their religious seriousness, and theological doctrine, however sincerely held, is not. Two people could disagree completely about the existence of God, the authority of scripture or the nature of an afterlife and still recognize each other as serious about ethics, provided each actually acted on a shared commitment to human dignity and mutual obligation.
That argument let Adler do something unusual for an American religious founder in 1877. He built Ethical Culture as a community explicitly open to people who disagreed about theology, on the reasoning that requiring agreement on doctrine before people could worship, or work, together was itself the thing dividing communities that a shared ethical commitment could otherwise unite. A member could hold onto Judaism, Christianity, or no religious belief at all, and still belong, provided the ethical commitment was genuine.
Later Ethical Culture societies read Creed and Deed as their founding case for that openness, and the reasoning inside it, that shared action matters more than shared belief, remains the clearest statement of what still separates an Ethical Culture society from an ordinary congregation built around a common creed.