What is the foundational principle for salvation discussed in the preface of Shri Sai Satcharita?

The preface, located in Chapter 0, establishes a foundational principle central to the land of Aryavarta: there can be no salvation without the grace of a Sadguru. This belief is described as ancient, firm, and supported by Vedic scriptures. The text emphasizes that this principle has existed since Vedic times. To receive this grace and attain knowledge, one must engage in service to the Guru. As Dnyaneshwar Maharaj is quoted in Chapter 0, service is the very "doorway" through which one can access the "treasure of knowledge" held by the saints.