According to the text, what is the significance of a Guru's mantra, and which specific mantra is mentioned for devotees of Sai?

The text explains that the mantra of the Guru is a powerful spiritual tool that is attained with great effort and holds immense power. As detailed in Chapter 1, selfless devotees have tested this power, which can unknowingly guide them toward liberation. The scripture elevates this practice by stating that the holy name of the Sadguru serves as the devotee's Veda and scripture. Specifically, it identifies 'Sai Samarth' as the primary mantra, clarifying that it also functions as the devotee's yantra and tantra, making it a complete and all-encompassing spiritual discipline.