I don't know. That's the short answer. Some abuse survivors, often survivors with severe Dissociative Identity Disorder (DID), have memories of being abused in Satanic ceremonies. These memories are usually "recovered memories" - memories that emerge later in life, although some abuse survivors report continuous memories of Satanic ritual abuse - memories which they have always had.
What do these memories mean? Does having a memory mean that the events actually happened the way that we remember them? The answer to this question is certainly "no." Research has shown that memory is fallible. We do not record the events in our life on a recorder and play them back later. We have all experienced times when our memory of an event conflicted with someone else's. The brain sorts and filters a huge amount of incoming sensory material. It then decides which very small percentage should be stored for later retrieval, and only portions of this material are stored.
Abuse survivors who have these ritual abuse memories often remember very similar things. They remember adult figures dressed in certain clothes performing ceremonies, for example. I will not go into further detail about the common threads in these memories. Most of us are familiar with some of that these survivors report to have happened to them. They are typically hideous and gruesome memories that involve some of the worst kinds of torture and abuse imaginable.
Can these memories possibly be real? Again, I don't know. In the past therapists tended to divide themselves into camps on this issue. There was a camp of true believers who believed in the literal truth of these memories. There was another camp, represented most strongly by the False Memory Syndrome Foundation, which believed that all of these memories were false, and that they were probably implanted by therapists.
I have worked with a very small number of patients who have had such memories. The first time I heard of these memories from a patient I had no idea what was happening. I did know that I had not "implanted" the memories. They were completely alien to my experience and to anything we had talked about in therapy. I saw the need to get some training in this area and I sought it out.
The advice that I received at the time was that I should believe my patient and believe in the literal truth of her memories. (I have since come to believe that this is not the best approach - that it is best to acknowledge that we just don't know what these memories mean.) I was confronted with evidence that many other abuse survivors were telling similar stories, and I was told about "generational" Satanic cults which were networked and which coordinated the rituals around the world. I even heard whispers of an international organization called the Illuminati which coordinated these cults and which intended to take over the world in the year 1999 by taking advantage of these "programmed" Manchurian candidates which they created in these cults.
Whew. Was any of this remotely possible? I began networking with other professionals to see what they thought.

