Can you summarize the main themes of the Shrimad Bhagavad Gita as explained in the preface?

The preface does not provide a summary of the main themes of the Shrimad Bhagavad Gita. Instead, it specifically references a commentary on the Gita by Lord Samarth Sadguru Dnyaneshwar Maharaj. As described in Chapter 0, the author quotes four verses from this commentary to illustrate a single, powerful point: the necessity of serving saints and one's Sadguru with body, mind, and soul to attain true knowledge. The preface uses this reference to support its central argument about the importance of Guru service for salvation.