Rig Veda: ~ The Atman is the cause; Atman is the support of all that exists in this universe. May ye never turn away from the Atman the innermost self. May ye never accept another God in place of the Atman nor worship other than the Atman?" (10:48, 5)
The Brihadaranyaka Upanishad declares: "He who worships the deities as entities entirely separate from him does not know the truth. For the gods he is like a pasu (beast)". (1. 4. 10)
The Sages of truth are not religious Gurus or Saints. The religion and its mythological Gods and mythological Gods are nothing to with the ultimate truth or Brahman or GOD.
All sect based beliefs are dualistic and unphilosophical nothing to do with the ultimate truth or Brahman. In spirituality, the ultimate truth is God.
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 Krishna calls it "the supreme secret." Krishna points out that the 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.
Understanding what is God is a not so easy. Religious people can only imagine God based on their beliefs.
That is why 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.
No dualities, no differentiation. Only Atman exists.
The Bhagavad Gita: ~ “brahmano hi pratisthaham” ~ Brahman (God) is considered the all-pervading consciousness which is the basis of all the animate and inanimate entities and material. (14.27)
It proves that the all-pervading Atman, which is present in the form of the consciousness, is God. Thus worshipping the form based Gods is meant for the ignorant populace who are incapable of the realizing the truth, which is beyond the form, time, and space.
Lord Krishna Says Ch ~V: ~ “Those who know me in truth.".
~ Know me in truth means know the ‘Self’ without the form, time, and space.
There are two kinds of audiences - the ordinary ones who desire the transitory heaven and other pleasures obtained as a result of ritual sacrifices, and the more advanced seeker who seeks to know the truth beyond the form, time and space. The Bhagavad Gita is meant for the first audience, to help lead its followers along the way. The Advaitic wisdom is meant for those who wish to go beyond the form, time, and space.