For some people the upside down cross is a symbol of the occult, satanism, or even demonic interference — for others, it’s a completely wholesome religious symbol.
Because the traditional symbol of Christianity is a cross, it’s easy to see why people would use the image of the cross turned on it’s head as a symbol for satanism or as an anti-Christian statement. It has been used by punk rock, heavy metal, and hip hop performers as a means of expressing dissatisfaction with the status quo.
Pop culture has reinforced this meaning of the upside down cross. In horror movies like The Amityville Horror, Rosemary’s Baby, The Devil Inside, Paranormal Activity, The Conjuring, and The Conjuring 2 the reversal of the cross is presented as something of evil origin. However, this is the Hollywood version of events.
In the historical case The Conjuring 2 is based on (the Enfield Poltergeist case), for instance, there were no reports of crosses being turned upside down. Ed and Lorraine Warren who investigated the Enfield case fictionalized in The Conjuring were devout Catholics who were in direct contact with the Vatican. The would not have misunderstood an upside down cross as being a malevolent symbol in real life.
There are many who believe there is nothing sinister whatsoever about the upside down cross. You may have even seen the Pope sit on a throne with an upside down cross carved into it. This is not because the pope is the anti-christ. This is because in the Catholic church, the upside down cross is called the Cross of Saint Peter or the Petrine Cross.
St. Peter’s cross originates (obviously) with St. Peter, who was crucified on an upside down cross. St. Peter requested the unusual crucifixion because he did not feel he was worthy to suffer the same death as Jesus (a normal crucifixion). Because the pope is viewed as part of St. Peter’s legacy (St. Peter was the “rock” Jesus built his church upon), the upside down cross can by a symbol for the papacy.
The upside down cross is used to decorate churches and has special meaning — but there is no negative symbolism. Despite what you might think at first glance (or might “learn” from a horror movie), there is no traditional “evil” association with an upside down cross. It’s just another old religious symbol that looks creepy to people who aren’t familiar with it. 
