Dear Ra:
I apologize for the very long delay answering your question.
When you ask about the "Grail Movement" I presume you are not talking a Catholic apostolate by the same name. Instead I presume you are talking about the movement inspired by the work of Oskar Ernst Bernhard originating in Germany in the late 1940s.
The motivation to write the "Grail Message" (The Light of Truth) is stated:
“I wish to fill in the gaps which have so far always remained unanswered in the souls of men as burning questions, and which never leave any serious thinker in peace, if he honestly seeks the Truth.”
His published bio states in part:
ABD-RU-SHIN (civil name Oskar Ernst Bernhardt) was born on April 18, 1875, in Bischofswerda, Germany. After being educated and trained in business, he established himself in Dresden. In the years that followed, he made many journeys abroad and wrote a number of travel books, stories, and plays.
Mr. Bernhardt’s travels took him through numerous countries in Europe, the Orient, and North America, bringing him into personal contact with peoples of many classes and cultures.
After residing for a time in New York, he journeyed to England, moving to London in 1913. There, the outbreak of World War I took him unawares. On account of his German citizenship, he was imprisoned in 1915 and interned on the Isle of Man for the duration of the war.
The seclusion of internment brought with it an inner deepening. After returning to his homeland in the Spring of 1919, Mr. Bernhardt soon became conscious of his task: to open the path for mankind to a new knowledge of Creation. In 1923, he began writing spiritually-themed essays & lectures under the pen name “Abdruschin” and later “Abd-ru-shin”, and by 1937, the lectures which constitute the Grail Message were completed.
Like all people who claim to "open the path for mankind to a new knowledge of Creation" Bernhardt is either delusional, under demonic influence, or both.
This group seeks the "Holy Grail," (a chalice) which they describe in their FAQ:
3. What is the Holy Grail?
Its actual existence is confirmed and explained by Abd-ru-shin in the Grail Message.
In order to comprehend these explanations, one should familiarise oneself with the thought that it is not only on earth that everything has been formed, but that is in the beyond in higher than earthly regions, albeit there somewhat finer and lighter, one could say that is more ethereal.
The Holy Grail is a chalice, Which is to be found in the highest plane of Creation at the border to the Divine and forms the only connection between God and the Creation. The Grail is the point of power transfer for the radiation-power emanating from the Divine. Just as the heart Allows the blood to pulsate through the body, then streams the creative power once a year through the Holy Grail into Creation to Strengthen it with new power. This is the day of the power-outpouring!
"higher than earthly regions, albeit there somewhat finer and lighter, one could say that is more ethereal"? "Highest plane of Creation"? "at the border to the Divine and forms the only connection between God and the Creation"? "The Grail is the point of power transfer for the radiation-power emanating from the Divine"?
These ideas can be found among many esoteric and New Age groups. It is an attempt to be spiritual without acknowledging the True God.
As with many cults and esoteric groups, even in the first centuries, there is a attempt to connect with the Christ and his people. This group attempts to connect their occult power with the Holy Spirit (My comments in red):
Every year on 30th May the Festival of the Holy Dove is celebrated. This Festival is closely associated with the well-known Event of Pentecost, but above all with the activity of the Holy Grail and the Holy Spirit. The Dove is the visible Sign of the Holy Spirit, of the Will of God.
Abd-ru-shin explained that the “Grail” and the “Grail Castle”, familiar from numerous myths and legends, do in fact exist, though not as an earthly vessel or edifice, but as the issuing-point of the sublime Creation-Work.
The Holy Grail is the Power Transmission Point between the Divine Realm and Creation. Through the Holy Grail at the appointed time all that exists is enlivened afresh with the Creator’s Power.
[There is no "transmission point" between God and His Creation and certainly not through a material object of a Chalice. ]
”It is the moment in which the Love of the Creator radiantly flows forth in the Temple of the Holy Grail, bringing new life and a new urge to create, pulsating downwards and diffusing itself through all the Universe. A trembling and a holy awe, with forebodings of joy and great happiness, vibrate through all the spheres. Only the spirit of earthman still stands aside, without intuitively sensing what is happening particularly to him at that moment; or in what a dull-witted manner he accepts such an immeasurable gift, because the limitations he imposed upon himself through his intellect no longer permit him to grasp such greatness!
[Here he makes a vain attempt to describe in his occult system something akin to the Mass and the Eucharist]
It is the moment when a new supply of vital energy is sent out into the entire Creation!
It is the necessary, ever-recurring ratification of the Covenant between the Creator and His Work!”
[the ever-lasting (not recurring) ratification of the Covenant is the Mass, which is the self-same sacrifice of Jesus 2000 years ago.]
This regular, rhythmic Event of the Outpouring of the Creator's Living Power did not gain entry to the Grail legends just as a happening affording “miraculous sustenance”, but there is a connection – not generally known – with Pentecost, when at one time after the death of their Lord – so painful to them – the disciples of Jesus were filled with special spiritual Power.
[Here he admits to "grail legends" to which he seems to want to incorporate. In the last analysis, to use a modern reference, this is the theology and archaeology of Indiana Jones]
Jesus had announced this experience to His followers with the words: “You will receive the Power of the Holy Spirit, which will come upon you!” This, however, was not a solitary event concerning only the disciples, but a general, ever-recurring event which the band of disciples was able consciously to experience in devout attunement.
[The indwelling and sealing of the Holy Spirit is not a "recurring" event, but a specific and intimate event the Holy Spirit gives personally to all who are baptized, and also at confirmation.]
“The festival of Whitsuntide [“Whitsuntide” is another designation for “Pentecost”.] has been instituted by Christians in memory of this occurrence, without their ever suspecting that at this approximate time each year the Day of the Holy Dove occurs in the Grail Castle, that is, the day of the renewal of power for Creation by the Holy Spirit. Naturally this does not always correspond exactly with the Whitsuntide holiday as figured out on earth, but at its approximate time!”
The bottomline is that the Grail Movement and the Grail Message is another false philosophical and pseudo-religious movement that leads people away from the True God, who is the God of Abraham, Isaac, and Jacob (not Ismael), who has revealed himself through the Bible and through His Church, the Catholic Church headed by King Jesus' Prime Minister, the Pope.
Stay away from this.
God Bless,
Bro. Ignatius Mary