Although they share the same theology (one omnipresent God, not a three-in-one trinity), rely heavily on the philosophy expounded by Ralph Waldo Emerson, and are congregation-run organizations that honor the many spiritual paths as all having value and do not believe anybody goes to a place called Hell, the three groups are really very different.
The Unitarian-Universalist Association is totally unrelated to either of the other associations and is generally unaware of them…. It’s a merger of two long-established “liberal” American denominations: the Unitarians, which have their origins in the work of a 16th century Spanish monk named Servetus, consist of essentially independent congregations which “affirm” a set of principles and acknowledge a “transcendent mystery” but many of which make a point of never using the words God, Jesus, or even Spirit; and the Universalists which is defined by the doctrine that all people must go to Heaven, as a loving God would never permit anyone to experience the eternal suffering we call Hell. The two associations merged in the 1960s and their congregations are usually located near colleges and universities, with active environmental and social justice agendas.
Unity and Religious Science (Science of Mind, Centers for Spiritual Living) are both considered part of the New Thought Movement, though there is no formal relationship between them. Their founders were students of Transcendentalism and mental healing methods and were influenced by Mary Baker Eddy’s Christian Science. Both groups focus on positive thinking and the use of affirmations and denials as part of releasing old thought patterns and replacing them with a New Thought: a mental framework that will lead to a greater experience of well-being. Unlike Unitarians or Christian churches that focus on Jesus’ “social gospel,” both groups encourage people to change the world from within, rather than focusing on or attempting to “fix” issues in the world around them.
Unity, however, was founded as the School of Practical Christianity and tends to be more Bible-based than what are now called Centers for Spiritual Living, which were originally called centers for Religious Science and were deliberately not based in any one religion. The focus in Unity is on the healing power of silent, affirmative prayer, while Religious Science Practitioners are trained to do written and oral treatments to help people undo “error thought” patterns and replace them with knowledge and experience of a higher Good.
Unitarian-Universalists are generally quite wary of the religious language of Unity and the metaphysical assumptions of Science of Mind. Religious Scientists are generally uncomfortable with the focus on the Bible, Jesus, and Christ Consciousness in Unity.
When I speak to a Unitarian-Universalist congregation, I generally emphasize the scientific and cultural aspects of my work; in a Unity congregation I draw on heavily on Fillmore’s work, and in a Center for Spiritual Living, I draw on Emerson, Hopkins, and Holmes to help make my points.