This card represents the mysteries and the unknown world of the spiritual realm. She is the intuitive, mystical priestess that can tell you your destiny. This card reminds me of the high priestess at Delphi in Greece, that would sit and channel messages for the many who would make the pilgrimage to see her. High on the gases emitted from the earth she would tell their future.
When the high priestess appears in your reading, as it is a major arcana card, and comes immediately after the magician, it signifies spiritual messages or an awakening into this unknown world. Expect the next phase of life to involve a deeper understanding of what lies hidden behind the meaning of what is presented to you, seeing things from a deeper perspective.
