The Humanist challenge to traditional Unitarianism and Universalism took on a geographical and historical character. The Humanist congregations tend toward the West (meaning West of the Hudson River) while the Liberal Protestants were stronger among the older and more established churches of New England, many of which pre-dated the Unitarian controversy of the early 19th century.
The conflict, which became known as the humanist-theist conflict, was sharp and protracted. Congregations split, members left, ministers lost their careers, stained glass windows were removed or covered up, hymns re-written, liturgies changed.
But before either denomination formally split, the controversy was defused by a turn toward a pluralistic agnosticism. Both sides tacitly agreed that no one knew the whole truth, and that therefore, multiple and competing theological perspectives could co-exist in the same worshipping community.
The merger of the AUA and UCA took place well into the history of the humanist-theist conflict. By 1961, Humanism was dominant in both denominations, but in a tense series of negotiations, just enough room for Liberal Protestantism was preserved in the new denomination to maintain the support and participation of liberal Christians and other theists.
Merger cemented the consensus: The prevailing theological perspective was an "agnostic pluralism" and UU liturgy was conducted, in most places, with a humanist language set. A small minority of congregations continued to use Liberal Protestant or otherwise theistic liturgies. The humanist language set became the official language of UUism at the time of merger and ever since.
The turn toward Pluralistic Agnosticism set the stage for Unitarian Universalists to be inspired a wider diversity of theological perspectives: mysticism, world religions which were included in the original Sources statement. The inclusion of the 6th Source established an institutional process by which additional Sources could become official acknowledged.
While the diversity of theological perspectives has become a centrifugal force in Unitarian Universalism, it is has been offset by the centripetal, or unifying, force of our public theology. Our public theology is our theological understanding of our work in the world, or mission. Our public theology is expressed in the Seven Principles, which have also continued to be developed over time. The most important development in the Seven Principles has been the further commitment to become an anti-racist, anti-oppressive, multicultural religious movement.
Comment: This is what I consider to be a thumbnail sketch of the story behind the Sources statement. This history, however, is only a part of a larger history, which I think needs to uncovered. The question is this: how does the story we tell about ourselves hide the decisions that were made to keep black religious liberals out of power in the UUA?