What Upanishadic shrutis describe the guru as essential to brahmavidya?

The provided corpus does not contain direct support for specific Upanishadic shrutis describing the guru as essential to brahmavidya — please consult a qualified spiritual guide.

However, the retrieved scriptures explicitly establish the Sant and Aksharbrahma guru as indispensable for attaining liberation and realizing the divine. Bhagwan Swaminarayan taught that when God is not physically manifest, a realized Sant serves as the essential refuge for the jiva.

"when God is not manifest on this earth, one should seek the refuge of the Sant who has the realisation of God" — Vachanamrut Vartal-10

The guru parampara tradition further codifies this through the Akshar-Purushottam siddhānt:

"Through the Aksharbrahma guru, Bhagwan always remains present with all of his divinity and bestows utmost bliss." — Satsang Diksha shlok 107

Accepting such a guru requires internalizing his virtues and maintaining unwavering association:

"Has one accepted a guru? If a guru has been accepted, his virtues should develop in us, should they not?" — Swamini Vato 17.28

For researchers, this demonstrates that the tradition prioritizes the realized guru's association as the practical means to spiritual realization, grounded in scriptural mandate rather than abstract philosophy.