When exactly did Unitarian Universalism become a post-Christian religion? Was it immediately after the UU merger in 1961? What religions were included when Unitarian Universalism became post-Christian? Was it mostly just Christians and Humanists, or were there others as well?

I often hear it said that our Seven Principles and Six Sources are not a creed, but a covenant. I guess I'm having a hard time understanding the difference. The Seven Principles and Six Sources are a statement of what Unitarian Universalists accept to be true or, in the case of the Sources, what they accept to be valid ways of knowing, correct? Can someone please explain to me how the Seven Principles and Six Sources aren't a creed?

Can someone please give me a brief overview of the history behind how atheists, humanists, agnostics, and deists ended up a part of these two Christian denominations? Did some Unitarian minister or Universalist minister just wake up one day and come out to the congregation as a humanist? How, exactly, did the incorporation happen?

We often hear people talk about the Unitarian roots of Humanism, especially individuals like John Dietrich and the Unitarian signers of the Humanist Manifesto I. What doesn’t get talked about as much is how Humanism became part of the Universalist tradition. Does anyone have any insights or resources I could use to learn more about Universalism’s historical relationship to Humanism?