According to ancient principles, why is having a Sadguru considered essential for a person's salvation?

The preface of the Shri Sai Satcharita, as detailed in Chapter 0, posits a firm and ancient belief that there is no salvation without the grace of a Sadguru. This principle is not a modern idea but has existed since Vedic times and is supported by Vedic scriptures. The text emphasizes that a Sadguru can manifest in any form, regardless of caste, religion, or age—they can even be a book, parent, or friend. This underscores the indispensable role of a Guru's grace in the spiritual journey toward liberation.