Lord Krishna confesses that the ancient wisdom of India the Advaitic wisdom has been lost.+

Lord Krishna confesses that the oldest wisdom of India the 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 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 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.

The pursuit of truth is all about accepting uncertainty and open questions and seeking the things that are a certainty. Believers and nonbelievers will not accept anything other than their inherited belief system or accepted truth.  Atheists feel there is no proof of God's existence. So the atheist chooses to just believe there is no such thing called God. And religionists just believe in the existence of their conceptual god.  A Gnani is fully aware of the fact that the existence or non-existence of God cannot be proven on the base of the physical self.  Therefore, a deeper understanding is necessary to understand and assimilate the truth about God's existence to know what is supposed to be God.

In Vedas, God  has been described as:~

  Sakshi (Witness)

  Chetan (conscious)

  Nirguna (Without form and properties).

  Nitya (eternal)

  Shuddha (pure)

  Buddha (omniscient)

   Mukta (unattached).

 ~ This indicates the nature of the Atman,  the Self.

 Thus,  the Atman the innermost Self is the ultimate truth or Brahman. Brahman is God

It means the ultimate truth is God.  That is Atman is Brahman.  That is the true self is the ultimate truth.

In Advaita Vedanta:~ Brahman is without attributes and strictly impersonal. It can be best described as infinite Being, infinite Consciousness, and infinite Bliss. It is pure knowledge itself, similar to a source of infinite radiance. Since the Advaitins regard Brahman to be the Ultimate Truth, in comparison to Brahman, every other thing, including the material world, its distinctness, the individuality of the living creatures, and even Ishvara (the Supreme Lord) itself are all untrue. Brahman is the effulgent cause of everything that exists and can possibly exist. Since it is beyond human comprehension, it is without any attributes, for assigning attributes to it would be distorting the true nature of Brahman. Advaitins believe in the existence of both Saguna Brahman and Nirguna Brahman; however, they consider Nirguna Brahman to be the absolute supreme truth. 

Atman is the true Self.   The true Self or Atman is in the form of consciousness and is the ultimate truth or Brahman or God in truth.  

 The Atman is hidden by the mind (universe), and the mind (universe) is within the Soul. The universe is within the Soul, the Self, and the Soul, the Self is within the universe. The chicken is in the egg, and the egg is in the chicken. The tree is in the seed, and the seed is in the tree. The same thing that was told by the Vedas was repeated by Saint John (10-38)? Father is in me, and I am in the Father. Thus the father (soul, the innermost self) is our immediate, neighbor- Love thy neighbor.  Therefore, one has to love the Soul to become one with the formless Spirit(Soul), which is the creator, sustainer, and dissolver of the world/duality.  

Therefore, Self-realization is necessary to realize the ultimate truth or god.  Self-realization is the real God-realization.

Brihadaranyaka Upanishad says:~ In that unitive state there is neither father nor mother, neither worlds nor gods nor even the Scriptures. In that state, there is neither thief nor slayer, neither low caste nor high, neither monk nor ascetic. The Self is beyond good and evil, beyond all the suffering of the Human Heart

Meher Baba said: ~ Give no importance to creed, dogma, caste, or the performance of religious ceremonies and rites.

The only Real Existence is that of the One and only God Who is the Self in every (finite) self.

The only Real Love is the Love for this Infinity (God), which arouses an intense longing to see, know and become one with its Truth (God)

The only Real Sacrifice is that in which, in pursuance of this Love, all things—body, mind, position, welfare, and even life itself—are sacrificed.

The only Real Renunciation is that which abandons, even in the midst of worldly duties, all selfish thoughts and desires.

The only Real Knowledge is the Knowledge that God is the inner dweller in good people and in so-called bad, in the saint and in the so-called sinner. This Knowledge requires you to help all equally as circumstances demand without expectation of reward, and when compelled to take part in a dispute, to act without the slightest trace of enmity or hatred; to try to make.

The real love is the love for the Soul, the Self. Thus ‘Love  Soul, the real Self'.:~Santthosh Kumaar