Lord Krishna confesses that the oldest wisdom of India (Advaitic wisdom) has been lost: people misinterpret and falsify it today as they did then.+

All sect-based beliefs are dualistic and unphilosophical nothing to do with the ultimate truth or Brahman.  In spirituality,  the ultimate truth is God in truth.

Lord Krishna confesses that the oldest wisdom of India ( 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 Lord Krishna calls it "the supreme secret."Lord  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.

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.

Sage Sankara: ~VC- Let erudite scholars quote all the scripture, let Gods be invoked through sacrifices, let elaborate rituals be performed, let personal Gods be propitiated---yet, without the realization of one‘s identity with the Self, there shall be no liberation for the individual, not  even in the lifetimes of a hundred Brahmas put together (verses-6)

The theosophist’s idea of the universe appearing and dissolving, days and nights of Brahma, entering into pralaya, etc. is intended for mediocre intellects, who cannot rise to the truth. 

It is a convenient fable representing the philosophic truth that the whole universe dissolves in your mind in deep sleep, thus entering pralaya, and rises again the next morning, i.e. it is all imagination, idea. The Brahman -God has nothing to do with it.

Although Sage Sankara puts the mystic goal highest in his mystical books, he is careful to say that this goal leads to Brahman, not that it is realization. :~Santthosh Kumaar