How does one achieve spiritual knowledge and salvation according to the ancient principles mentioned in the Shri Sai Satcharita's preface?

The preface to the Shri Sai Satcharita, found in Chapter 0, emphasizes the ancient and firm principle from Vedic times that salvation is unattainable without the grace of a Sadguru. Drawing from the wisdom of Dnyaneshwar Maharaj, the text explains that the path to knowledge is through dedicated service to the saints. One must serve with their entire being—body, mind, and soul—surrendering at the Guru's feet without ego. This selfless service is presented as the doorway to the treasure of knowledge, which enlightens the heart and dispels all doubts. It is this unwavering service that is said to lead to the Guru's grace and, ultimately, the attainment of spiritual knowledge.