You might not have heard of Theosophy, but there's a good chance you've felt its impact.
Theosophy was a "new religious movement" founded in New York City in 1875 primarily by Henry Steel Olcott and Helena Blavatsky. They were interested in Hinduism and Buddhism (which they learned about from books) and believed that they were telepathically guided by beings called Mahatmas.
In 1879, they travelled to India to learn about Hinduism, and the following year they travelled to Sri Lanka where they apparently became the first Americans to convert to Buddhism. But despite taking vows of refuge, their primary allegiance was to Theosophy, not Buddhism, and Olcott was known to disparage the local Buddhist beliefs as being inferior to his understanding of the Buddhist religion.
Olcott remained in Sri Lanka where he became an important figure in the local revival of Buddhism, mostly by funnelling money into scores of Buddhist schools. (Buddhism had suffered in Sri Lanka until British rule.)
Blavatsky died in 1891 and Olcott in 1907, and the Theosophy movement was taken over by Annie Besant. In 1909, Besant took custody of an Indian boy named Jiddu Krishnamurti (over the objections of the boy's father) and proclaimed the boy to be Maitreya, the foretold "Messiah" of Buddhist scripture. As an adult, Krishnamurti distanced himself from the Theosophy movement, but remained a popular author and orator until his death in 1986.
Despite some early schisms and paedophilia scandals, Theosophy was a great success, and by the 1920s they had tens of thousands of members across the United States, Europe, Australia, and New Zealand. Apparently many of them could be found in the British suffragette movement. D.T. Suzuki, the Japanese author and philologist who translated multiple Buddhist sutras into English, was a Theosophist.
I'm not sure I understand Theosophical beliefs well enough to summarize them in a fair way. They believed in reincarnation, karma, and single divine "Absolute". They believed that there was an ancient universal "true" religion that modern religions were incorrect corruptions of. Many of their leaders claimed to be clairvoyant. They also had some odd racial theories.
Despite having the hallmarks of a religion, the Theosophists denied that they were a religion and insisted that they were a philosophy, and claimed to eschew any code of ethics.
The movement went into decline after 1929 when Krishnamurti departed the organization, apparently repudiating claims of his Messiahhood. But the Theosophists are still around, still promoting their ideas, and they very well might have a presence in your city. It is often said that Theosophical ideas were influential in the "New Age" movement which started to take off in the 1970s (and which was definitely still a thing when I was in university in the 1990s).
To me, the biggest legacy of the Theosophists is they created confusion in the West about what Buddhism is, by deliberately conflating their organization's beliefs with Buddhism. I still encounter this confusion all the time. If you ever hear someone say "Buddhism is more of a philosophy, not a religion", you're probably hearing the echo of the Theosophists.
I'm not sure why the Theosophists were so successful around the turn of 20th century. I guess it points to a certain dissatisfaction amongst Westerners with their own religions (a dissatisfaction that would continue throughout the 20th century with the decline of Christianity in the West), and a hope that they would find more satisfaction in Eastern religions.
But unfortunately, Theosophy was not an Eastern religion. It was a Western religion dressed in Eastern garb.