Spiritualism was popular in the 19th Century U.S., largely due to the general belief in superstitions and the high mortality rate, especially of children. The families needed to believe that their loved ones weren't gone, but just in another form and could be contacted. Many of the same mediums did seances for Abraham Lincoln and his wife, as well as John Wilkes Booth and his family. The comments of John Wilkes Booth around one medium about planning to kill Lincoln really concerned the medium. However, he didn't want to give away his client, so he kept warning Lincoln during the seances that he was in great danger (Alford 2022). Starting in 1857, Spiritualism was aligned with the Free Democratic Party, often called the working man's party, but slavery being abolished would result in competition with jobs between the working men and the free slaves. As a result, Republican's ideals about ending slavery were closer to the spiritualist's aims and they became aligned politically with the Republicans as of the election of 1860. Some Spiritualists called themselves Christian Spiritualists and would set up lyceum-type meetings in which invited speakers would present lectures. At its height, some 300,000 Spiritualists existed and used the assistance of the famous Fox sisters and others for seances (Lause 2016). Lily Dale in upstate New York, was the origin of the Fox sister seances and the town did a thriving trade catering to tourists interested in seances well up into the 20th Century. One medium actually taught Harry Houdini some of the seances' most famous tricks. Besides seances, mesmerism and hypnotism also flourished (Wicker 2004).
In Victorian Britain, Spiritualism was just one of the many nontraditional beliefs that existed between the Orthodox Christian Faith and Atheism. These include scientific Christianity, Rosicrucian, Buddhism, and Spiritualism. Many individuals believed and participated in various alternative faiths at the same time as being Christians, or any other alternative beliefs, including Mesmerism (Franklin 2018).
The best way to explain Aleister Crowley's magic is to start with an Egyptian myth. Osirus, the sky god, married Isis, an earth goddess and their brother, Seth was jealous, so he killed Osirus by tricking him into a box and putting the box into the Red Sea. Isis negotiated the body of her husband back from another king who found it, and Isis had sex with Osirus' corpse and gave birth to Horus. Crowley thought that there were we ages of humankind. The mother goddess was first, then in the first century AD the new era was the sky/father god and now in the twentieth century it was the age of Horus, the adolescent son. He thought magic was an action that caused change. He didn't believe in good vs bad gods.
The next step was his new wife started channeling a god on their honeymoon and this god led Crowley to take down a book of laws, which was told to him by a voice. Add in Heroin and Cocaine addiction and a general bisexuality and you have a new religion with spells that could be sex magick or rituals or whatever. He liked to be mysterious, and his mom used to call him 666 or devil child, he had a wild sense of humor, and you have the origin of the mistaken assumption he worshiped the devil.
For some people who studied psychology and physics, especially during the above-mentioned time of popular Spiritualists, the purpose was not to talk to so-called spirits through a medium, but to actually prove life after death, as a separate plane of existence. William James, brother of the famous Henry James author of "The Turn of the Screw" was a Psychiatrist, they would say an Alienist or a Keeper of an Asylum, and a medical doctor and he searched for scientific proof of this other plane. Interestingly, after his death, he showed up through a medium, unlike Houdini, who never did. He and other physicists looked for proof of this secondary plane in many different ways, including the evolution as possibility that the spirit world had evolved out of ours. They also believed that the early mediums were real, but the Charleton mediums ruined their reputation. They tried hypnotizing patients and proving a secondary physical source of the psychic phenomenon. I recommend you read Deborah Blum's book listed in the reference section of this page, as well as Alford's "In the Houses of their Dead" also in the reference section if you want a nuanced view of 19th century American and Victorian development, beliefs and pursuit of the spirits of their dead. It's more like the movie "The Haunting in Connecticut" than the various paranormal shows searching for ghosts on TV.
Alford, T. (2022). In the houses of their dead : the Lincolns, the Booths, and the spirits (First edition.). Liveright Publishing Corporation, a division of W. W. Norton & Company.
Black, J. (2022). The game is afoot : the enduring world of Sherlock Holmes. Rowman & Littlefield.
Blum, D. (2007). Ghost hunters : William James and the search for scientific proof of life after death. Penguin Books.
Chéroux, C. (2005). The perfect medium : photography and the occult. Yale University Press.
Davies, O. (2007). The haunted : a social history of ghosts. Palgrave Macmillan.
DuQuette, L. M. (2003). The magick of Aleister Crowley : a handbook of the Rituals of Thelema. Weiser Books.
Franklin, J. J. (2018). Spirit matters : occult beliefs, alternative religions, and the crisis of faith in Victorian Britain. Cornell University Press.
Lause, M. A. (2016). Free spirits : spiritualism, Republicanism, and radicalism in the Civil War era. University of Illinois Press.
Lachman, G. (2005). A dark muse : a history of the occult (1st Thunder Mouth's Press ed.). Thunder’s Mouth Press.
Ptacin, M. (2019). The in-betweens : the spiritualists, mediums, and legends of Camp Etna (First edition.). Liveright Publishing Corporation, a division of W. W. Norton & Company.
Schmitt, J. Claude. (1998). Ghosts in the Middle Ages : the living and the dead in Medieval society. University of Chicago Press.
Tompkins, M. L. (2019). The spectacle of illusion : deception, magic and the paranormal. Distributed Art Publishers, Inc.
Wicker, Christine. (2004). Lily Dale : the town that talks to the dead (1st HarperCollins paperback ed.). Harper, SanFrancisco.
Winkowski, M. Ann. (2007). When ghosts speak : understanding the world of earthbound spirits (1st ed.). Grand Central Pub.
Yeats, W. B. (William B. (2015). A vision : the revised 1937 edition (M. M. Harper & C. E. Paul, Eds.). Scribner.