Nonduality does not need the support of any Scripture or Revelation like the Veda

Without Sage, Sri, Sankara there is no Advaita (nonduality).  Since it was mixed up with orthodoxy there is lot of confusion. I am highlighting all the obstacles, which is blocking one from realizing the ultimate truth or Brahman. There are so many non-dualistic masters of the east and also from the west who expound Advaitic or non-dualistic knowledge but none of them are helpful to reach the ultimate end. 

Yoga Vasishtha :- "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."

Those who assert the world is not illusion because they are unaware of the fact that, the world is the reality within the illusion (waking). And they are frightened to consider the world as illusion. Without considering the world as illusion, it is impossible to acquire Self-knowledge or Brahma Gnana or Atma Gnana. 

After verifying through deeper inquiry if one finds the world is the reality within the illusion then he cannot again say the world is not an illusion. 

If one is frightened to accept the world is an illusion (waking) then he is unfit to acquire Self-knowledge or Brahma Gnana or Atma Gnana. Those who stuck with the reality of the world, are stuck with the reality of the individual experiences of birth, life and death, which takes place within the unreal world. Thus the pursuit of truth is for those who have courage to accept the reality as it is, that is the reality without form, time and space. 

According to Adavaita Vedanta, the Veda addresses itself to two kinds of audiences - the ordinary ones who desire the transitory heaven and other pleasures obtained as a result of ritual sacrifices, and the more advanced seeker who seeks to know Brahman. Thus, the purva mimam.sa, with its emphasis on the karma kanda of the Vedas, is meant for the first audience, to help lead its followers along the way. However, the Vedanta, with its emphasis on the jnana kanda, is meant for those who wish to go beyond such transient pleasures.

 
Sri, Sankara says in Brahma Sutras: that Brahman is the cause of the world, whereas in Mandukya he denies it. This is because he says that at the lower stage of understanding, the former teaching must be given, for people will get frightened as they cannot understand how the world can be without a cause, but to those in a higher stage, the truth of non-causality can be revealed. 

Thus those who are frightened to accept the world is mere illusion should never indulge path of wisdom. 

Modern Ramanuja- J.Krishnamurti describes to perfection is the awakening to Reality—the realization that pure Consciousness alone is, that the perpetually fluctuating and evanescent contents of the mind derive from it. This awakening effectively happens in an instant. But in order for the lightning flash to take place, resulting in a firm and unshakable certitude, a long labour is necessary, which he seems to underestimate. “Truth is a pathless land” is his answer.

Many thinkers feel J.K’s teaching seems essentially negative, a potent but bitter medicine for those imprisoned by institutional cults. He breaks the student’s bonds, but then leads him to a vast desert where he abandons him. The ultimate state of consciousness he describes is that of the traditional sage or a fully enlightened being, but he does not show us the process leading to the realization of this state. He describes marvellously the goal, but does not indicate the steps to be taken: his recurring phrases "unified consciousness" and "let go" are not a road-map.

J.Krishnamurti‘s approach was more practical, and he stuck with the reality of the world, he took it as real.

Sage Sri, Sankara himself has warned us not to use ambiguous words, and to practice semantic analysis in his book "Definition of one's own Self. (Page 199, v.24 of "Sankara's Selected Works)

Sage Sri, Sankara founded his Advaita Vedanta either on reason independent of Sruti or on Sruti confirmed by reason." Sri, Sankara's commentary on the Mandukya Upanishad, II, 1: This (the unreality of duality) is borne out by the Sruti ... But it is possible also to show the unreality of the object world even from pure reasoning, and this second chapter is undertaken for that purpose.

Sage Sri, Sankara himself had often said that his philosophy was based on Sruti, or revealed scripture. This may be because Sri, Sankara addressed the ordinary man, who finds security in the idea of causality and thus with the idea of God—and Revelation is indispensable to prove the latter. He believed that those of superior intelligence, have no need of this idea of divine causality, and can therefore dispense with Sruti and arrive at the truth of Non-Dualism by pure reason. 

Sage Sri, Sankara, in debates with Buddhists and others who did not recognize the authority of the Vedas, had been obliged to prove the truth of the  Advaita by means of reason alone. Mandukya Upanishad: a scripture which appealed to reason to the exclusion of Revelation. 

Nonduality does not need the support of any Scripture or Revelation like the Veda. For it is based, not upon the varying theological fancies, which are as numerous as the sands of the sea, but upon reason, the common heritage of all mankind, irrespective of colour or creed or clime.

Upanishad says:-

Katha Upanishad:-This Atman cannot be attained by the study of the Vedas, or by intelligence, or by much hearing of sacred books. It is attained by him alone whom It chooses. To such a one Atman reveals Its own form. (Ch.-II -23-P-20)

Mundaka Upanishad: - This Atman cannot be attained through study of the Vedas, nor through intelligence, nor through much learning. He who chooses Atman—by him alone is Atman attained. It is Ataman that reveals to the seeker Its true nature. (3 –page- Upanishads by Nikilanada)

When the Upanishads declares:  It is attained by him alone whom It chooses. To such a one Ataman reveals its own form. Then what is the use of indulging through our intelligence or our accumulated knowledge, when one is not chosen by the self, which is the soul, which is in the form of consciousness.

There is no need for any philosophy in pursuit of truth because they take the seeker nearer to the truth but they create more doubts and confusions. 

The scriptures and theories and teaching based on the ego are not the yardstick. Using them as yardstick to understand and assimilate the truth will lead one towards the pursuit of arguments. Seeker of truth has to discover on his own, the truth of his true existence by inquiring “what is mind?” and “what is the substance of the mind?” and move forward. 

The ultimate truth is one without the second, the one is not in the sense half or two, but the one that remains forever One, without the second. The consciousness is all pervading. There is no place where consciousness is not. 

Consciousness is in everyone, consciousness is in everything, consciousness is one behind many. Consciousness alone is. It means the universe and its contents are the visible form of consciousness. And consciousness in turn is invisible form of the universe, which appears as mind. 

As the ego becomes more and more inward-turned, it becomes gradually freed from external desires, and when all such desires are fully eliminated self-realization is completely freed from obstruction.