The Eternal Sadguru is the messenger of Supreme being. Brahma Vidya (the Divine Wisdom) emanates through Him and he is the Lord of the spiritual domain. In the beginning of the creation, he is sent by the Supreme Being into the world with the light of spiritual knowledge. His only mission is to awaken the suffering souls of the world from the sleepy state of ignorance, by shedding upon them the divine light. He liberates the craving and deserving souls from the bondage of Prakriti and enables them to enjoy the nectarine bliss of the Supreme Being. When the current of the spiritual knowledge of the Eternal Guru flows in the soul of a human being, the status of Rishi or Maharshi is attained. The knowledge of Brahma Vidya or Vihangam Yoga, which is found in the Veda, the Upanishads and other holy books, is solely due to him. He has been addressed as Sukrit Deo, Atri, Jyoti, Aj and the Sanatan in the Vedas. He has also been mentioned in the Upanishads as Amanav Purush. The Eternal Sadguru sermonizes the spiritual practitioners and the real devotees in his manifested or un-manifested form. He can take a human form at his will.
सद्गुरू नित्य अनादि है, स्वयं सिद्ध गुरू आय |
सिद्ध गुरू अभ्यास से, परमपरा गुरू आय || स्वर्वेद 2.2.12
Sadgurū nitya anādi hai, svayaṁ sid’dha gurū āya |
Sid’dha gurū abhyāsa sē, paramaparā gurū āya || – Swarved 2.2.12
When humans become too much engrossed in the allurements of Prakriti and tend to deviate from the true path of spiritualism, the Eternal Sadguru appears in the world in a human form to salvage the souls. Sometimes he inspires a liberated soul to take a human form and propagate the divine knowledge of Vihangam Yoga (synonym of Brahma Vidya). Such a great soul possesses full spiritual knowledge right from his birth and is called the Swayam Siddha Sadguru. Sometimes the Eternal Guru appears before an industrious yogi and guides him to perfection. Such a great yogi is granted the authority to act as a Sadguru. Such a Sadguru is called the Abhyasa Siddha Sadguru. There is only one Sadguru at one period of time that carries the authority of the Eternal Sadguru to propagate Brahma Vidya (Vihangam Yoga). Once a Soul has been authorized by the Eternal Sadguru to act as a world preceptor (Jagadguru), he prepares his progeny or any other deserving disciple to carry forward the science of Brahma Vidya generation after generation. Such a succeeding Sadguru qualifies to become Parampara Sadguru only after perfecting Vihangam Yoga and getting authority from the Eternal Master, again.