Essence of the Mandukya Upanishad on Non-Duality:

"Brahman Alone Is Real"

(Al Drucker’s summary of commentaries by Shankara, Gaudapada and Nikhilananda on the Mandukya Upanishad):

     To believe that there can be anything other than Brahman (the eternal Absolute), is sheer ignorance. This ignorance is the cause of every possible sorrow. But ignorance is not real. In fact, every teaching and every sadhana (spiritual practice) deals only with dream activities, which, ultimately, are just illusory mind concoctions. In Truth, every being is already free and forever remains free, equal to Brahman in every way and not different from Brahman in any way. There is neither bondage nor ignorance which must be removed, or can be removed by spiritual practice. Every teaching and every sadhana is just a dream effort, much like taking dream water to slake dream thirst. Its purpose within the dream is to remove the mistaken notion which obsesses the dream character, that he is anything other than the ever-pure and unchanging Brahman. Sadhana dehypnotizes him from the hypnotic idea that there can ever be anything other than Brahman, that there can be a world or individuals that are real. But separate individuals are not real and were never real.

  All efforts and teachings within the dream have relative value only for relative beings. They have no real value in Truth, because the ever-free, ever-awake Atma (the true Self) is not affected by any dream effort, and the illusory, asleep, dream character will not transcend the dream as a result of it. Along with the awakening, when it plays out within a dream of awakening, the story takes on the character of spiritual awareness and spiritual effort. In such a dream, awakening and sadhana are linked to each other, with sadhana appearing to be the cause of the awakening. But in Reality there is no such connection, because these activities have no real existence, and therefore, nothing is happening. It is just that in the story playing out within the dream, the heightened spiritual effort and awareness engaged in by the individual appear to be the cause of awakening. But, though sadhana and awakening seem to be related, they are much like a crow landing on a coconut palm just as a coconut happens to fall. There is no causal connection between them. They are coincidental happenings within an illusory story, appearing to be causally related.

    In Reality neither sadhana nor awakening has any effect or any meaning. There is ever only one meaning to everything, and that is Brahman. Reality has nothing to do with dreams of awakening, or of individuals and their experiences, or of a world in which they exist separate from Brahman. Reality is Brahman alone. There is nothing else. Brahman is not the cause of anything. It does not create. It does not transform Itself into unreality. It does not become something else. It ever is all there is. Illusory dreams may play out for a moment, appear and disappear like passing clouds. You may believe you are an individual and that you and others inhabit a world extending all around you. But that is total delusion. It cannot be. All there ever is is Brahman and only Brahman. It alone is who you are. It is the One Self, the Atma, which is another name for Brahman, and therefore, exactly the same as    Brahman. Atma has nothing at all to do with individuals, bodies, minds, or circumstances. It is you, the real you. You cannot not be It. You cannot be anything else. You were never born. You’ll never die. You remain as   Brahman alone. Unchanging. Unchangeable. And that is all there is. Ever.