The earliest one I could find was Eugène Vintras(page only available in french) .
Vintras was a French Gnostic revivalist from the around the middle of the 19th century (1807–1875).
He preached the end of the world, and claimed he had received messages from the Archangel Michael and that he himself was a reincarnation of Elijah. Vintras was condemned by the Vatican, and after his condemnation he started using an upside down cross. The mention of this being satanic comes from Éliphas Lévi (an occultist himself) stating that Vintras' inverted cross was "indicative of satanic influences"
Cult and Ritual Abuse: Narratives, Evidence, and Healing Approaches, 3rd Edition
By James Randall Noblitt Ph.D., Pamela Perskin Noblitt
Vintras, following further attacks from the Church, adopted the
inverted cross as the symbol of the new dispensation of which he was
the prophet, inverted because the Reign of the Suffering Christ had
been superseded by the Reign of the Holy Spirit of Love.
Blood in the City: Violence and Revelation in Paris, 1789-1945
By Richard D. E. Burton, Professor of French and Francophone Studies Richard D E Burton
image courtesy of Wikipedia
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