In May 1819 Channing travelled to Baltimore to preach at the ordination of a young minister, and used the occasion to state plainly what the liberal party in New England Congregationalism actually held. The sermon was printed, circulated in enormous numbers, and turned a loose tendency into an identifiable body.
Its method is as important as its content. He begins with how scripture is to be read: as a book written in human language, in particular circumstances, to be interpreted by reason like any other, and not as a set of propositions immune to the judgement of the mind that reads them. From that he argues against the Trinity as unscriptural and unintelligible, against the doctrine that the Father's wrath required a substitutionary sacrifice, and against the inherited total depravity of human nature.
The positive claim underneath is moral. God's goodness must mean something recognisably like what goodness means when applied to a person, or the word is empty and worship becomes submission to power. A doctrine that requires us to call good what we would condemn in a human being is, on that test, disqualified. That argument is the reason the sermon still gets read, and it is the reason it was received as an attack: it makes the moral sense of the ordinary hearer a court that theology has to satisfy.