When did the "upside down cross" become a symbol of anti-Christianity? I can find no mention of it in the Bible. However there is, in Christian history, the story of Peter's execution – which really isn't a sign of anti-Christianity but the admiration he had for Jesus. Is there a line in the sand date when this became an anti-Christian symbol?
Eugène Vintras (1807–1875) may be the first to use the inverted cross in a distinctly anti-Christian way. He was a Gnostic revivalist operating in France during the middle of the 19th century. He preached the end of the age, and claimed to have received messages from Michael the Archangel and that he himself was a reincarnation of Elijah. He was condemned by the Vatican, and subsequently began using an upside-down cross:
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symbol of the new dispensation of which he was the prophet, inverted because the Reign of the Suffering Christ had been superseded by the Reign of the Holy Spirit of Love.1
His masses reportedly included "chalices magically overflowing with blood," and occultist Eliphas Levi considered the inverted cross that Vintras wore as "indicative of satanic influences":2
The practices of Vintras's followers were even more bizarre, and the symbol thereafter began to be associated with the occult more generally, such as in a 1891 novel, Là-bas:
His costume was a long robe of vermilion cashmere caught up at the waist by a red and white sash. Above this robe he had a white mantle of the same stuff, cut, over the chest, in the form of a cross upside down.3
It's also used in a 1903 depiction of 17th century occultist Étienne Guibourg (link is possibly an occasion for sin). In the (cropped) image below, he is shown wearing an upside-down cross as he performs a Black Mass during the Affair of the Poisons:
I haven't found any indication that Guibourg actually wore an inverted cross, but rather that he was only depicted that way once Vintras's use of it became associated with occultism more generally.
If one includes Gnosticism within Christianity, then perhaps Vintras's use of the inverted cross would not be considered "anti-Christian." However, at the very least, his use of it and his perceived involvement with the occult caused the symbol to become generally associated with Satanism by the end of the 19th century.
- Richard D. E. Burton, Blood in the City: Violence and Revelation in Paris, 1789–1945, 164
- James Randall Noblitt, Cult and Ritual Abuse: Narratives, Evidence, and Healing Approaches, 161
- Joris-Karl Huysmans, Là-bas, Chapter XX
Apparently the inverted cross first was associated with evil in Bloodwolf’s book “ Chaos Lord." Antiwolf jpg
But modern Satanists may have chosen it as a symbol of anti Christianity due to the use of that symbol on the album cover of a Black Sabbath album in the 1960's
Most of this is from memory going back a few years and really is nothing more than my thoughts from having looked into it some time ago.