Question: "What is the meaning of an upside-down / inverted cross?"
Answer: The upside-down cross, or the inverted cross, is a symbol with a long history and, in modern times, divergent meanings. As its name implies, an upside-down cross is simply a Latin cross turned so that the crosspiece is closer to the bottom than the top. A person hanging on such a cross would be positioned head-downwards.
For centuries, the inverted cross was considered a Christian symbol, based on an ancient tradition that the apostle Peter was crucified upside down. One version of the story says that Peter, facing martyrdom by crucifixion, requested that his cross be inverted because he felt unworthy to die in the same manner as Christ. The upside-down cross, sometimes called the Cross of St. Peter or the Petrine cross, thus became a symbol of humility. The inverted cross is sometimes associated with the pope, who Catholics believe can trace his authority back to Peter. Artwork featuring the Petrine cross may contain an overlay of the “keys of heaven,” based on Matthew 16:19.
Recently, however, it is common for the upside-down cross to be used as a symbol of atheism, humanism, and the occult. Several black metal bands use an inverted cross to call attention to their supposed devotion to Satan. Upside-down crosses appear in horror movies such as The Omen and The Conjuring as a signal of demonic activity. The symbol crops up in tattoos, on pendants, and as a logo on t-shirts. Sometimes, it is accompanied by statements such as “Believe in Yourself,” “There Is No God,” “Black Mass,” or “Not Transformed.” In these contexts, the obvious intent of the inverted cross is to declare an opposition to Christianity. Turning the cross upside down becomes a means of denying the truth of Christ and mocking His sacrifice.
So, what is the meaning of an upside-down cross? It depends on the context. When the symbol is used in a church setting, it is most likely a reference to Peter and the manner of his death. In other contexts the inverted cross is often an anti-Christian symbol. In our fallen world, holiness is often mocked and what is good and pure is twisted by the “god of this age” (2 Corinthians 4:4). The cross, a symbol of Christ, is turned upside down to become a symbol of the devil.