The sources are mostly from french here. I will obviously translate it as much as I can
What is a guru ?
According to linguists, the sanskrit term guru (serious) is a 'cognat'* from latin gravis that derivates from a indo-european radical meaning heavy. So the guru is a 'big man'.
The term gu means darkness and ru means lights that disperses, so guru means the one that disperses the darkness — Advayataraka Upanishad, verse 16.
In the Vedānta trend (an orthodox trend), the Vedāntasāra in XVIth century gives as definition:
a real guru is a man for who the practice of virtues is familiar,
who pruned all the roots of evil with the sword of wisdom [...] who
behaves with dignity and independence. Who sees gold and precious stones
with the same indifference as scrap iron and shards of glass, who
totally cares to push back the darkness of the ignorance where other
people are benighted Wikipedia source in French
*cognat: word of a language that has the same origin as another word of another language
The role of a guru
His role in Hinduism is to guide his pupils to the relief.
Adi Shankara, from the Advaita Vedanta trend, explains what is the role of the guru and the qualities he must get:
When the teacher finds from signs that knowledge has not been grasped or has been wrongly grasped by the student, he should remove
the causes of non-comprehension in the student. This includes the
student's past and present knowledge, want of previous knowledge of
what constitutes subjects of discrimination and rules of reasoning,
behavior such as unrestrained conduct and speech, courting popularity,
vanity of his parentage, ethical flaws that are means contrary to
those causes. The teacher must enjoin means in the student that are
enjoined by the Śruti and Smrti, such as avoidance of anger, Yamas
consisting of Ahimsa and others, also the rules of conduct that are
not inconsistent with knowledge. He [teacher] should also thoroughly
impress upon the student qualities like humility, which are the means
— Adi Shankara, Upadesha Sahasri 1.4-1.5
The teacher is one who is endowed with the power of furnishing arguments pro and con, of understanding questions [of the student],
and remembers them. The teacher possesses tranquility, self-control,
compassion and a desire to help others, who is versed in the Śruti
texts (Vedas, Upanishads), and unattached to pleasures here and
hereafter, knows the subject and is established in that knowledge. He
is never a transgressor of the rules of conduct, devoid of weaknesses
such as ostentation, pride, deceit, cunning, jugglery, jealousy,
falsehood, egotism and attachment. The teacher's sole aim is to help
others and a desire to impart the knowledge.
— Adi Shankara, Upadesha Sahasri 1.6
Wikipedia source in English
How to be a guru ?
There is no authority able to entitled a guru; hinduism has not a centralized authority (like Catholicism) to do this. Jean Varenne (a French Indologist) tells "It is inconceivable someone self-proclaims himself a guru, because this is the public acknowledgement, about the self-fulfillment of the person concerned, who tells that he has the stature of a guru".
Then, there are two possibilities:
- To be recognized as a guru by a guru (guru shishya parampara). So the guru tells his student is able to teach: so to be a guru.
- The rumor plays its role: the guru is someone people listen/learn from. If there are people claiming they are disciples, then his cause is listened, it makes him a guru.
Of course, this is above all based on ethics. Indians know the existence of fake gurus and fake sadhus. To illustrate this, a low-relief (VIIth century) in Mahaballipuram shows a cat in meditation position while mouses, mistaken by cat's attitude, approching the cat by trust, will finally die.
Source wikipedia in French
The texts mentionning the neccesity of a guru
The term guru appears in post-vedic Upanishads. So this is only Smriti, not Shruti.
Alexandre Astier et al. call the Shvetâshvatara Upanishad as "the front door of hinduism". Written during the VIth and IIIth century BC, it is composed of 5 chapters, and gives three new elements that seem to be an annunciation of what is the next hinduism. In other words: the line break between Vedism and Hinduism.
- The idea of a supreme God (theism, a trend to merge all Gods of vedic pantheon to a only God). [...]
- The texts insists on the necessity of a guru in charge of the transmission/teaching of wisdom and the necessity of a guru as an
intermediate to reach the relief.
- The knowledge is not enough for the âtman to realize its merge with brahman, the grace of the divinity (prasâda) is needed too.
Source: L'Hindouisme, Alexandre Astier, ed. Eyrolles, 2013, Chapitre 6: les nouveaux textes, page 79-80.
The Advaya Taraka Upanishad; classified as Upanishad of Yoga; describes in details the mystical experience that happens when you managed to obtain the vision of the supreme reality. This same texts clearly mentions:
A guru totally fulfilled is needed to obtain the supreme reality vision.
This guru must be permeated of Vedas,[...], liberated from any antipathy, expert of yoga, devotes himself permanently in the purity of the spiritual union.
source: Les 108 Upanishads, traduction et présentation, Martine BUTTEX, ed. Dervy, 2012
be careful some internet free content about Upanishads are very short compared to original version
When one tells every Hindu has a guru by default : his father, this is in opposition to what is written in Upanishads. A guru is not anybody, so not your father except if he is a real guru.
Besides Upanishads mention the necessity of a guru to reach the relief. It does not say you're not hindu if you have not a guru. It just say you will not reach the relief without a guru
Moreover, not everybody, even hindus, is in quest of the relief. Some spiritualities are "poorer" but are still spiritualities.
So no Shruti tells this and not all hindus are in quest of the relief and it does not make them nastika in my opinion.