St. Peter's Cross vs. Inverted Crucifix
Okay, so yeah. I did some googling. An upside down cross is not a Satanic symbol. It’s a St. Peter’s cross (St. Peter chose to be crucified upside down), which is a Christian symbol. If there is a corpus, however, it is not a St. Peter’s cross; it’s an inverted crucifix. An inverted crucifix is used as a symbol for the anti-Christ (there’s a thing with Satanists where you invert religious symbols to get their antithesis: a crucifix is Christ so an inverted crucifix is the anti-Christ, Christ died at 3 p.m. so 3 a.m. is the devil’s hour, etc.) during Satanic black Masses. So where an upside down cross is either a Christian symbol or a symbol that you’re a super edgy hipster who doesn’t do his/her research, an upside down Crucifix is apparently a symbol used by actual desecrate-the-Eucharist, goat-skulls-and-demon-worship Satanists (and apparently also super edgy hipsters on Tumblr).