Why Guru is essential for spiritual sadhana ?

The word “guru” which is randomly used for a teacher today is Not the Vedic word for a teacher.
The Vedic term for the teacher is Acharya (is the one who imparts Knowledge)
‘ GURU’ means the one who leads you from darkness to light.(gu= darkness ra = light ,+ u = move forward , put together is guru)
and THIS “GURU “ is within YOU., and an Acharya introduces you to your internal guru.
GURU in you reveals the TRUTH (gyana )
“ Guru Bramha Guru Vishnu, Guru Devo Maheshwara, Guru Shakshath, Parabramha, thas mai shree guruve Namaha” meaning … hey GURU who is in the form of Bramha, Vishnu, Maheshwara, ARE the VERY (saksath)Pra Bramha (supreme Bramham ) that which is within me, I SALUTE TO YOU.
This is the first shloka that is taught when one embarks on learning and repeated at the beginning of any class.
There have been no external gurus; even the great Sankara was addressed as Achraya not a Shankar guru
Eg Dronacharya, Kripacharya Madhavacharya, etc were ACHARYAs NOT Gurus.
In the Veda, from what I have read and “seen” so far, the human agency is minimal. Agni and Indra are the dominant powers that lead the aspiring souls through and beyond the inner continents of bhur, bhuvah, and svaha. The foremost Guru on this path of Self-Knowledge is thus Agni. The host of Gods follows in his wake, Indra, Vayu, Maruts, Vishnu, and more.
It is in the Upanishads, we have the start of what we would conventionally understand as a guru or teacher or guide.
With that as context, a Guru is one who has acquired knowledge of the Self and its mechanics or is the very Power/Consciousness that we aspire to. A human intermediary attains the status of a Guru, not merely by reading the Veda or Upanishads but actually realizing the states of consciousness they allude to. And this Guru has the power to manifest and transmit this power within the mind and consciousness of an aspirant.
Realization and not mere book knowledge is what a true Guru guides the child’s soul towards. To someone who teaches anything else, including the highest material knowledge, the term Guru will not apply in the Vedic/Upanishadic sense.
Gu means darkness, and Ru means glare/light, it means from darkness to light is the work of Guru guru is the power who can provide from darkness to light off the path to their disciples and so far as the Satya Sanatan religion information is concern Guru is the most powerful and respectful than parents and other distinguished personality. If we talk about lord Shiva Ganesha Brahma Vishnu Mahesh Durga Allah KABIR Satya Purush, all these become great only because of the group h so just come into the life of greatest Guru of this earth Sant Rampal Ji Maharaj yes he is a mine of time and can into You All The spiritual power more than getting more than Gods and other temporal power and also you can attend third supreme solvation Satya Loka that can be shot by Christianity Mohammedans forest Jainism Hinduism and most of the powerful farmers and saints but now it is possible into the life
Between the Vedic and Upanishadic ages the science of Self-Knowledge has moved from its luminous freedoms to the less free method of systematic inquiry, codification, and the intervention of illumined human agency. Not to say the Vedic age had no Gurus but a human agency seems to have been less relevant to the acquisition of Self-Knowledge. The arrival of the Deva was paramount and not the intermediary.
The word Guru Purnima also indicates that Guru removes the darkness-ignorance like the full Moon at the night. Thus, Guru is compared to Sun and Moon, who remove the darkness-ignorance. There is a difference between Guru and Satguru. Both preach the correct knowledge only. But, the knowledge preached by Satguru is complete and the knowledge preached by Guru is incomplete.
Guru is a human being, well-versed in scriptures. Satguru is the God in human form. The word ‘Guru’ denotes the human form. The word ‘Sat’ denotes God. The meaning of the very word ‘Guru’ means the preacher, who removes the darkness-ignorance by giving the knowledge-light.
If the knowledge is correct and complete, it generates tremendous force, which is called devotion. Devotion (Bhakti) is the necessary force that transforms knowledge (Jnana) into practice (Karma).
Thus, devotion is generated by knowledge. If the knowledge is correct but incomplete, it generates insufficient force and hence, the knowledge is not transformed into practice. If the knowledge is correct and complete, it generates sufficient force that transforms the knowledge into practice.
Complete knowledge means the elaborate form of knowledge that contains the answers to all the possible doubts. When you hear such knowledge, no doubt will arise in your mind because the knowledge already contained the answer for every doubt. When the right knowledge is complete, the realization is also complete and in such a case, no force can stop the transformation of knowledge into practice.
Therefore, the knowledge of Satguru is always elaborate and such elaboration is the meaning of the word Vyasa. The knowledge of sage Vyasa is always elaborate. This Guru Purnima is also called Vyasa Purnima, the birthday of the sage Vyasa. Vyasa is considered to be the incarnation of Lord Vishnu, God. This day falls in the rainy season, in which the dark clouds cover the Sun or Moon in the sky. This indicates the strong doubts of the people, who receive the knowledge.
We perform the Guru Purnima on this particular day. This is not the correct concept. Any day can be Guru Purnima on which you receive the correct and complete knowledge from Satguru. The sunrise happens to be 5.30 am in summer and 6.30 am in winter. Thus, there is no fixed time of sunrise. Whenever the sun rises, that is the sunrise.
Similarly, whenever the Satguru gives the knowledge, that time is Guru Purnima. We always give importance to a specific place and specific time and ignore the event. We should give importance to the event. Wherever the event takes place, such a place is sacred.
Whenever the event happens, such time is auspicious. People perform the ceremonies of their departed parents on specific days. They do not give value to the person (Bhakta) available on that day. Whenever a valuable person is available, the ceremony should be performed. The fruit flies in the availability of the valuable person and not on the date and place.
You must have the mood to receive the knowledge from Satguru whenever the Satguru has the mood to deliver the knowledge. The mood of Satguru to deliver the knowledge is important and not the mood of the receiver to receive the knowledge. Whenever the cloud rains, the farmer should plow the field. Whenever the farmer plows the field, the cloud will not rain. Generally, Satguru gives the knowledge when you do not have the mood to receive it. It is a test for you. Whenever you have important work, the Satguru delivers the knowledge.
He is testing you to see whether you give the top most important knowledge or to other works. When you don’t have any work to do, then you will have the mood to receive the knowledge. This means that you give the lowest importance to Satguru and His knowledge. Krishna preached the Gita when Arjuna was involved in the war.
When the war was over, Arjuna asked Krishna to repeat the Gita since he has full leisure [time] without any other work. Krishna refused to deliver the Gita saying that He has no mood. Therefore, whenever Satguru comes to your house, that day is Guru Purnima. Whenever Satguru delivers the Divine knowledge, you must stop all the other works and receive the knowledge with full attention. Then only you have understood the significance of Guru Purnima.
The main program of the human incarnation is only to deliver the right and complete knowledge to the human beings so that they will practice it and attain salvation. The fruit will not come without practice. The spiritual sadhana will not come without devotion. The devotion will not come without the correct and complete knowledge.
Devotion and practice are automatic consequences of the correct and complete knowledge. Therefore, Shankara told us that if you receive the correct and complete knowledge, salvation is automatic (Jnaanadevatu kaivalyam…). The Gita also starts with Jnana yoga only. Therefore, if you can catch Satguru, salvation is an automatic consequence. If you catch Satguru, you are bound to receive the correct and complete knowledge from Him. Therefore, Guru Purnima is the top of the festivals in the year.