Anybody can write a commentary on the Upanishads and other great philosophic texts, all they need to do to pour out words.+

Religious Gurus and yogis are the theologians, the Gurus and yogis are religious heads, and these people are not Gnanis.   Some great scientists and professors claim themselves who have mastered Vedas and Upanishad and these people are Gnanis but intellectuals. These people dominate the ignorant populace, and they are not the Gnanis ~ they are not Gnanis because they have accumulated bookish knowledge. All their accumulated knowledge is second hand.

Tripura Rahasya: ~ Second-hand knowledge of the Self-gathered from books or Gurus can never emancipate a man until its truth is rightly investigated and applied; only direct realization will do that. Realize yourself, turning the mind inward. ~ 18: 89

Some Gurus even simply go on fabricating beautiful words and theories and creating an illusion in the minds of people that perhaps they are authentic seers, enlightened people.

When the Vedas and Upanishad declare that the consciousness or Atman is actually nothing but Brahman, then why go round and round, by various tortuous paths, like the blind led by the blind.

Ish Upanishad says:~ Those people who have neglected the attainment of Self-knowledge and have thus committed suicide.~ 10/11/12

Bhagavan Buddha: ~  There are only two mistakes one can make along the road to truth: not going all the way... and not starting.

Sage Sankara was right in saying: ~  VC~ 58. Loud speech consisting of a shower of words, the skill in expounding the Scriptures, and likewise erudition - these merely bring on a little personal enjoyment to the scholar but are no good for Liberation.

59. The study of the Scriptures is useless so long as the highest Truth is unknown, and it is equally useless when the highest Truth has already been known.

60. The Scriptures consisting of many words are a dense forest which merely causes the mind to ramble. Hence men of wisdom should earnestly set about knowing the true nature of the Self.

61. For one who has been bitten by the serpent of Ignorance, the only remedy is the knowledge of Brahman. Of what avail are the Vedas and (other) Scriptures, Mantras (sacred formulae), and medicines to such a one?

Fortunate is the one who does not lose himself in the labyrinths of philosophy. 

Yoga Vasistha:~ "Teachers, interpretations of sacred texts, the force of religious merit--none of these lead to the realization of that Ultimate Truth which is revealed in the clear reflection of the heart, engendered from contact with the good."

Anybody can write a commentary on the Upanishads and other great philosophic texts, all they need to do to pour out words. Some westerners along with some orthodox pundits translated and published books. The influence of Sage Sankara’s wisdom is nothing to do orthodox belief systems.

Some philosophers in the past dissented from this interpretation of Vedanta philosophy, holding that the incarnated Souls were separate from the Divine Essence and only finally merged with it after the cycles of birth. All these theoretical philosophies are based on the imagination based on the false self (ego or you) within the false experience (waking).

It is quite easy to do this even by those who have never understood the Upanishads but think they have. Thus the dualists and their disciples have written commentaries on scriptures: But all these have written to satisfy themselves, not to get the truth because all their interoperation are based on the dualistic perspective. The truth will reveal only when one is able to reason on the nondualistic perspective.

Likewise, thinkers and poets of the Age of Devotion (Bhakti) of the 16th century believed in a God with attributes who became very tangible when incarnating as Avatar and was attainable simply through love and devotion rather than scholastic and intellectual meditation.

For them, the Bhagavad Gita became the main vehicle of inspiration with its qualified and deistic Monism, rather than the scholastic and esoteric path shown by Sage  Sankara.

The truth-seeker has to reject the devotional path if he wants to realize the truth beyond the form, time, and space. The path of Bhakti is for the ignorant populace. The only path of wisdom leads the seeker of truth on his journey to the ultimate realization of the true nature of the Universal Essence, which is the Soul. The Soul is present in the form of consciousness. : ~ Santthosh Kumaar