What key themes are presented in the opening sections of Chapter 18 regarding the Sadguru and the dissemination of spiritual knowledge?

The beginning of Chapter 18 presents several key themes. Firstly, it establishes the supreme nature of the Sadguru, hailing him as the eternal truth of Brahman and the 'Ruler of Maya' who teaches the knowledge of the Self. Secondly, it describes the transformative impact on a devotee, using the analogy of a salt doll dissolving in the ocean to represent the merging of the individual self with the Guru. Finally, as Chapter 18 progresses, it argues against the secrecy of spiritual teachings, asserting that a Guru's instructions, even those received in dreams, should be shared for the enlightenment of all, much like the sage Budhakaushika shared the Ram Raksha.