Cross of St. Peter
An inverted cross symbol is the cross of St. Peter, who, according to tradition, was crucified upside down because he felt unworthy to die the same way as Christ.
As Catholics believe the pope to be a successor of St. Peter, the inverted cross is frequently used in connection with the papacy, such as on the papal throne and in papal tombs. It also symbolizes humility because of the story of Peter.
The inverted cross has more recently been appropriated as a symbol meant to oppose or invert Christianity, such as in some branches of Satanism.