The Unitarians started in the fourth century when, at the Council of Nicea, the Trinitarians won the vote. Those priests who believed there was only one god were given the choice of converting or excommunication. The Unitarians went underground for many centuries and as organizations that are persecuted often do, they attracted other people with splinter beliefs and were accepting of variation in ideas. In the sixteenth century they were able to go public in Transylvania. By the nineteenth and early twentieth centuries they were pretty open. In fact, in the 1930s a group of free-thinkers which included a couple of dozen Unitarian ministers got together and formed the Humanist Association (see Humanist Manifesto I). By the mid-twentieth century the Unitarian philosophy was attracting large numbers of young people and they were growing rapidly.
Meanwhile, in the nineteenth century a group of liberal Methodists who didn’t like the constant threats of eternal damnation for even minor infractions broke away from that church and formed the Universalists. That is, they believed in universal salvation. God was too good to punish anyone forever, but would allow everyone to be saved eventually. By the mid-twentieth century the mainstream churches had moved away from the fire and brimstone approach so the Universalists didn’t have much different to offer. They had a lot of real estate and endowments, but they were losing members.
The Universalists approached the Unitarians and suggested a merger because they were both quite liberal, socially. Unfortunately, the Unitarian hierarchy saw the money they would bring with them and got greedy. They merged and became the Unitarian Universalists. Since then the Universalist mafia has slowly taken over the adminstration of the denomination, and the Unitarian schools of theology. There has been a shift to the theological right to the point that they are now not very far left, theologically, of the Methodists.
When I joined in 1968 the minister of the local church was an atheist. After he retired, they’ve gotten increasingly theistic ministers and only give lip service to humanism. I tried to resist the movement locally, but finally gave up and left.