In yogic or Nirvikalpa Samadhi, is just like swoon or deep sleep, where one is unaware of anything, not even the universe.+

Lord Krishna confesses that the oldest wisdom of India (our true Advaita philosophy) has been lost: people misinterpret and falsify it today as they did then. It is not yoga but philosophic truth. But nobody knows it. The teachers of philosophy and leaders of mysticism or religion do not want to inquire into the truth and have no time for it. (Gita ~Chap ~ IV-v.2)

In Gita Chap.IV where Lord Krishna says:- "This yoga has been lost for ages" the word yoga refers to Gnana yoga, not other yogas: the force of the word this is to point this out.

Lord Krishna describes some of the other yogas but devotes this chapter separately to Gnana Yoga. So one sees even in those ancient days people did not care for Advaita; they wanted religion; hence, Gnana got lost. That is why Krishna calls it "the supreme secret." Krishna points out that the yoga must-see is "Brahman in action."

Gita Chap.IV: "He who achieves perfection in Yoga finds the self in time." This means that after his yoga is finished, he begins the inquiry into ultimate truth, and in due course, this inquiry produces the realization of the universal spirit as the result.

Lord Krishna Says Ch. V:~ “Those who know the Self in truth.". The last two words (tattvataha) are usually ignored by pundits, but they make all the difference between the ordinary concept of God and the truth about God. 

Religious path and the yogic path are nothing to do with ultimate truth or Brahman. The scriptures are nothing to do with ultimate truth or Brahman. Gurus, swamis, and yogis are nothing to do with the ultimate truth or Brahman.  The experience of birth, life, death and the world are nothing to do with the ultimate truth or Brahman.  Individual God is nothing to do with ultimate truth or Brahman or God in truth. 

Karma or Action is nothing to with the ultimate truth of Brahman. Creation and creator is nothing to do with ultimate truth or Brahman or God in truth, because the soul, which is present in the form of consciousness, is the formless, timeless spaceless existence. Brahman means the ultimate truth. The one who has realized the ultimate truth is Brahma Gnani.

Advaitic or nondualistic truth is the rational truth, scientific truth, and ultimate truth.

Eradicating ignorance completely is necessary. And this is possible only through self-knowledge or Brahma Gnana or Atma Gnana.   Thus, there is no other road to freedom other than Gnana.  There is no other entrance other than Gnana.  The ignorance will vanish only when the non-dualistic or Advaitic wisdom dawns.

That is why Sage Sankara, indicated in Bhaja Govindam says:~  (Jnana Viheena Sarva Mathena Bajathi na Muktim janma Shatena) ~ one without knowledge does not obtain liberation even in a hundred births, no matter which religious faith he follows.

Then it is no use going a roundabout way, trace the Brahman which is the formless substance and witness of the universe, which is in the form of the mind.  By tracing the source of the mind or universe one will be able to realize the Brahman or God in truth.

Self-knowledge or Brahma Gnana or Atma Gnana is meant only for those who have an intense urge, and courage to accept the truth with humility and reject the untruth. Since people start comparing with their scriptural knowledge, it becomes impossible to assimilate and realize the non-dual truth.   Therefore, there is no need to convince anyone other than our own selves to get the firm conviction.

Sage Sankara:~  56. Neither by yoga, nor by Sankhya, nor by good work, nor by learning, but by the realization of one's identity with Brahman is Liberation possible, and by no other means.

Brih. Upanishad: ~ "Yoga does not yield truth or liberation." ~(page~32)

One who is in Samadhi will not know that this universe is consciousness, therefore yoga is not the means to Self- knowledge or Brahma Gnana or Atma Gnana. In Samadhi the yogi knows nothing, sees no universe; so if there be nothing whatever, how can the yogi know that the universe which is something is consciousness? If the universe is not seen in Samadhi than why is the word Atman and Brahman are used?

By shutting his eyes in Samadhi yogi does not know the universe, which confronts him. Hence, the universe can't be known as Brahman through yoga alone. One is in a non-dual condition in sleep or Samadhi, One without a second, true, but he did not know it at the time. You say only in the waking experience afterward. Hence, there must be an inquiry so that you find non-duality whilst you are awake so that you can see nonduality at the time not afterward. Hence, too the need for inquiring into the universe and knowing it as Brahman whilst one is awake, and not during sleep or Samadhi.

When can one say there is no error in his knowledge? When one sees all three states as consciousness, which is the true self; then there will be no doubt. Hence, one must see the beings and objects, if he is to see them as consciousness, the true self. But sleep and Samadhi does not show them to him. Hence, their knowledge is not perfect, not free from error and doubt.

The completion of the course of Yoga, whether Raja or Dhyana leads as Patanjali rightly says in sloka 1 to stopping all ideation. When this is attained, only one thing can then occur--the yogi will enter a condition like sleep.

"Like," because he will have done it by set effort and voluntarily, whereas ordinary man falls into sleep involuntarily and when Patanjali lists sleep as one of the five hindrances, he means that while a practitioner is still struggling and has not yet attained his goal of concentration, naturally to fall asleep prematurely is a sign of failure.

The ignorant emerging from deep sleep does not regard his body and his experience of the world as consciousness, which is the true self,  but he thinks he is an individual separate from the world. Hence, he feels duality as reality, similarly, to the yogi. The one, who does not see duality, can have no Gnana. 

A Gnani sees and says all this is an illusion made out of consciousness, which is the true self. Hence, there is unity in diversity, whereas, the yogi who enters Samadhi or sleep has none to start with.

Remember:~

Gnanic Samadhi is Sahaja Samadhi that is the natural state of the self. In natural states in which the Self is in full wakefulness, and then one asks what is meant by this universe, the universe is seen in this Sahaja Samadhi. Whereas Yogic or Nirvikalpa Samadhi is just like swoon or deep sleep, where one is unaware of anything, not even the universe. (See page 640,641 and verse 132,133 of Tattireya Upanishad and panchadeshi regarding this matter).

Lord Krishna confesses that the oldest wisdom of India (our true Advaita philosophy) has been lost: people misinterpret and falsify it today as they did then. It is not yoga but philosophic truth. But nobody knows it. The teachers of philosophy and leaders of mysticism or religion do not want to inquire into the truth and have no time for it. (Gita –Chap- IV-v.2)

In Gita Chap.IV where Lord Krishna says: ~ "This yoga has been lost for ages" the word yoga refers to Gnana yoga, not other yogas: the force of the word this is to point this out.

All other yogas are based on physicality but  Gnana Yoga is based on the Soul, the Self. So People did not care for Advaita; they wanted religion; hence, Gnana got lost. 

Gita Chap.IV:~ "He who achieves perfection in Yoga finds the self in time." This means that after his yoga is finished, he begins the inquiry into ultimate truth, and in due course, this inquiry produces the realization of the universal spirit as the result.

Lord Krishna Says Ch. V:~ “Those who know me in truth.". The last two words (tattvataha) are usually ignored by pundits, but they make all the difference between the ordinary concept of God and the truth about God.

Sage Sankara says:~ Yoga is not the means of liberation.

Brihad Upanishad:~ Even yoga cannot give perfect concentration and that the only way to gain it is philosophical realization. This confirms the Manduka Upanishads statement that yoga can no more succeed than the ocean can be emptied with a blade of grass. (p~133~first para)

In Sutra Bashya and Manduka: ~  Samadhi and sleep are identical.

Brihad Upanishad does not advocate Samadhi. People are going down, down, and down.:~Santthosh Kumaar