Actually, although some Satanist churches have adopted this icon much like the Third Reich adopted the Native American swastika, it isn’t a symbol of the devil nor does it mean one worships the devil. Firstly, if you’re referring to the crosses in the nail stamp picture, those aren’t upside down. Similar to one of the golden rules of good tattoos, images should face out and away from the body. So those crosses are actually “right side up”. If you want to get technical we can get into the infamy of the upside down cross originating with St. Peter and how he felt unworthy to be crucified in the same manner as Jesus. That is where the reverence to the Christian symbol of an upside down cross originated from, although he was not technically the first one the Romans crucified in this manner. ;) (I love theology plus have a bit of Catholic schooling in addition to other credentials in my past) But would it make any difference to you if I worshiped the devil or Jesus or the flying spaghetti monster? I’m an intensely spiritual non-preachy person.