As is true with all historic events, that Humanist Rebellion against Liberal Protestantism has a beginning, a middle, and an end. It is a story. It is, to a large extent, our story.
By simply listing brief summations of Humanism and Theism as differing options on a menu of theological perspectives, the Sources statement don't explain how their clash threatened to split liberal religion, and how that clash was defused and partially resolved.
I won't retell the whole story of the conflict, but will remind everyone that it was not a silly conflict. It was a serious dispute over the nature of reality that would define the teachings and liturgy of our religious communities. A lot was at stake: churches gained and lost members, ministers lost their careers, congregations were split. Beautiful stained glass windows were covered up or destroyed.
Ultimately, Unitarian Universalists moved toward a theology of pluralistic agnosticism about the propositions of formal theology. All opinions would be welcome, as no one really knows the whole truth. (The parable that best exemplifies the retreat from the battle lines is the story of the "Blind Men (sic) and the Elephant" which seemed to come into Unitarian thought with its re-telling by Sophia Lyon Fahs in "Long Ago and Many Lands" in 1948).
The Sources statement also exemplifies that Pluralistic Agnosticism: a menu of differing theological traditions that allows each person to pick and choose their truth.
The defusing the Humanist-Theist dispute by turning to Pluralistic Agnosticism had two major effects.
1. Pluralistic Agnosticism allowed UU's to acknowledge other spiritual and religious traditions beyond the binary of humanism and theism. These newly discovered, or re-discovered, and acknowledged influences are summed in the source statements about mysticism, world religions, and earth-centered spirituality. I suspect that as time goes on, more sources will be discovered and acknowledged in a statement of the influences on our living tradition: western Buddhism, Feminism.
2. But for all the growing diversity of theological influences on contemporary Unitarian Universalism, what unites UU's (as much as that is possible) is our public theology, as expressed in the Principles list, and in the 2nd Source: "words and deeds of prophetic people which challenge us to confront powers and structures of evil with justice, compassion, and the transforming power of love."
(Isn't it interesting that the Sources statement was explicit about the powers and structures of evil" which the Principles fail to describe what stands in the way of their realization?)