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.+

Sage  Sankara said:~ Neither by the practice of yoga nor philosophy, nor by good works nor by learning, does liberation come, but only through the realization that Atman and Brahman are one in no other way. (1) Vivekachoodamani v 56, pg. 25

Lord Krishna confesses that the oldest wisdom of India (our true Advaitic wisdom) has been lost: people misinterpret and falsify it today as they did then. It is not yoga but the philosophic truth. But nobody knows it. The teachers of philosophy and leaders of mysticism or religion do not want to inquire into 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 yoga must see "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. 

Thus, the yogi when he finds yoga is inadequate and useless to realize the 'Self' must drop yoga and take up the path of Gnana. 

Swami Vivekananda: ~ ‘Jñāna Yoga is divided into three parts. First: hearing the truth--that the Atman is the only reality and that everything else is Maya. Second: reasoning upon this philosophy from all points of view. Third: giving up all further argumentation and realizing the truth. This realization comes from being certain that Brahman is real and everything else is unreal.

Sage Sankara says:~ Yoga is not the means of liberation (page 132-133 - Commentary on Brhadaranyakopanishad.

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

Brihad Upanishad: does not advocate Samadhi.

Sage Sankara in the commentary to "Brahma Sutras:- " "The highest beatitude is not to be attained by Yoga." (Sacred Books of East Series page 298 Vol.1.) he also says Samadhi is the same as sleep (p.312) ---this indicates that yoga is not the means to Self-realization. And yogic Samadhi is not nondual Self-awareness. 

Panchadasi: - The impossibility of yoga arrives at a successful end to its practices. (P.509 v, 109)

Bhagavan Buddha gave up his austerities of yoga as impossible and useless. (Page.70/71 "Buddhism in Translation” by Warren)

Bhagavan Buddha got enlightenment only after he gave up Yoga. Unless one exercises his reason--there is no chance of getting the truth. 

There is no need for any practice to realize the ultimate truth or Brahman or God. Perfect understanding assimilation of ‘what is what’ is very much necessary to realize the ultimate truth or Brahman or God in truth.

Remember:~ 

Yoga does not yield truth, because it ignores the objective world.  Yoga has its place rather than its value and its value is for a certain type of mindset.  One cannot live without the physical world; it is the basis of his life, so it must be the starting point of his inquiry. Things, not imaginations, must be the seeker’s material.

Yogi shuts his eyes against the world and then has the temerity to declare that he knows the world to be Brahman! Because he has not inquired into it, he knows nothing.

Yoga helps the yogi by giving him the feeling that the world, which confronts him is not worth bothering about, it detaches him from the world; it makes him treat the world as a dream, i.e. an idea. It does the same to his ego to some extent, because he becomes indifferent to what happens to him. But the great secret is that this is only feeling, he feels these things only but does not know that the world is an idea. Such knowledge can come only after deeper self-search and in no other way. That is why yogi cannot be Gnani.

Sage Sankara says:~ Yoga is not the means of liberation (page 132-133 - Commentary on Brihadaranyakopanishad

Yoga can yield the only duality because everything that one can do or practice becomes a vanishing 'known.' It yields relative truth based on imagination, which is true from the physical viewpoint of view, not non-dual truth, which is the ultimate reality.

It is the difference between feeling and knowledge. The feeling of the yogi that the world is unreal may change in the future because all emotions are liable to change; and the fact is that yogis do change, as when they indulge in accumulating wealth they lose their sense of world unreality though previously they felt it.

 A permanent view of the world as unreal can come only after Soulcentric reasoning; such knowledge cannot change. Were the yogi of sufficiently sharp intellect he could discover the ideality of the world by Soulcentric reasoning alone and then it would not be necessary for him to have gone through yoga practice at all; that is why the yoga is for dull or middling intellects.

To realize the truth of the whole, one must know the world, that confronts him, otherwise, he gets half-truth. The seeker of truth should not run away from the external world means incapacity to think. Thus, it is necessary to know the nature of the world in which he exists.

The ultimate truth is attainable by perfect understanding, assimilation, and realization of ‘what is what’.  The perfect understanding of ‘what is what’ leads to Advaitic Self-awareness.  : ~ Santthosh Kumaar