According to the preface of the Shri Sai Satcharita, what is the fundamental principle for attaining salvation, and how is it achieved?

The preface, which constitutes Chapter 0 of the Shri Sai Satcharita, establishes a firm and ancient principle from Vedic times: there is no salvation without the grace of a Sadguru. To attain this grace, the text emphasizes the path of dedicated service. It quotes Dnyaneshwar Maharaj, who advises that if you wish to gain spiritual knowledge, you must serve the saints with your entire being—body, mind, and soul—and do so without ego. This service is described as the very doorway to the treasure of knowledge. Therefore, the fundamental path to salvation is through surrendering to a Sadguru and performing selfless service to earn their grace.