The inverted cross being an anti-christian symbol was something created almost entirely by hollywood media in the 60s to 80s. Before that point it was the cross of St Peter and was one of the accepted latin crosses. I say almost entirely, because it was used as an anti-christian symbol by a couple of culty-types in the 1800s, but they were extremely fringe, pulling random claims and practices from a hodge-podge of questionable occult knowledge, and their leaders were probably quite insane. Thanks for coming to my TED talk.
And also a major art festival used it as a symbol for St Peter in my city ages ago and made a lot of christians very shouty, which was funny, but got annoying after a while.
