system
A chat on 4chan
human
There’s also a theory that the Grail existed many thousands of years before Christ and that it was present in the Garden of Eden itself. It is portrayed as a stone cast down from heaven when Lucifer rebelled against God as mentioned in texts like Isaiah and Ezekiel. The gem was given to Adam when he lived in the Garden of Eden after the rebellion of the angels. When he and Eve were cast out, the “Grail” was lost to him as well. Seth, Adam’s progeny, was said to have gained re-entry into Eden and to have recovered the Grail. Seth remained in paradise for 40 years IIn chapter five of Genesis, you'll find that it is through Seth’s lineage that we arrive at the story of Noah and given that every other human being is snuffed out by the flood that makes Seth the father of humanity. So here the secret societies are guarding Seth’s bloodline and the magic stone. 

These alleged secret societies and gnostic sects that are into the Grail bloodline don’t believe that Jesus was the Son of God (just an important prophet whose story got corrupted by the Church and Rome) instead they are paving the ground for their own Messiah who will solve all the worlds problems (in Christianity this would be the antichrist) so the goal would be to destroy Christianity to pave the way for their Messiah and unite all world religions under one banner.
gpt
Lucifer’s stone is said to be the only object in existence that wasn’t made by Satan. It fell out of Lucifer’s crown during the war in heaven and fell to Earth. Maybe the secret societies were using the Grail story to cover up for the fact that they believed like the Gnostics that Lucifer was the God of material reality? The Cathars believed that Satan created the material world. 

Otto Rahn’s theory was that the Grail was cover for Cathar beliefs, rituals and initiation ceremonies. Cathars encoded their beliefs and practices in the Grail story and hid it of future generations and the Grail story wouldn’t arouse Catholic suspicions. Everything was symbolic and all the characters in the stories carefully chosen. Many of the poets of the Middle Ages were Cathars inspiring people with those stories and spreading their heretical beliefs that way. Chretien de Troyes the first man to mention the grail described it as being held by a woman using both of her hands. Graal the precise word de Troyes used, was an old French word for dish. Maybe the dish they are referring to is the one that held the head of John the Baptist? Maybe despite everything the Cathars secretly venerated John the Baptist like the Knights Templar? They did both make southern France their base of operations and many Templars came from Cathar families like Bertrand de Blanchefort the Templars refused to participate in the Crusade against the Cathars even when called on to do so by the Pope the only time they refused a direct order. It all comes down to the existence of an alternative religion to the Catholic Church that existed in secret using code and allegory to hide their beliefs from the mainstream.