Shakespeare and the Mysteries

Clarence tells us of King Edward's mysticism in these terms:"Hearkens after prophecies and dreams;And from the cross-row plucks the letter G."- Shakespeare, Richard III, I, 1.The Mysteries, wrapped as they are around the Alphabet, become revealed plainly with a bit of knowledge. In the above passage, the "Letter G" clearly maintains some relative import. Further, that Clarence is "hearkening"