B.G 17.23

By the directive "Om, Tat, Sat", the three aspects of Brahman is remembered. By this, the Brahmanas, the Vedas, and sacrifices were instituted in ancient times.

Gīta Bhāshya 17.12

The directive "Om, Tat, Sat" is an ancient directive as indicated by the word 'purā'. 'Om' - shows the all-pervading nature of Brahmna; 'tat' - brings out the ever-changing, yet constant nature of Brahman; and 'sat' - highlights he is endowed with all auspicious qualities and also the reality of existence. The word 'tena', i.e. by him, implies 'by Brahman'. Thus, the Vedas and sacrifices were instituted in ancient times by Brahman for worship.

Again, for the purpose of prescribing actions and others as a duty, the eulogy says "Om tat sat" and so on.

These are indeed the names of the Supreme Brahman:

"The universe is pervaded by the self-complete form, 'Om', as stated in the Vedas, without modification, 'tat', endowed with all auspicious qualities 'sat', and not by others. Therefore, he is referred to as 'Om Tat Sat'."

- thus indeed, it is mentioned in the appendices of the Rigveda.

The second part of the above verse refers to the meaning of 'tat', i.e. that, sound.

"Indeed, dear, this was 'sat' i.e. existence, in the beginning."

- stated thus as well

"'Om' is Brahman."

- stated thus as well.

The word 'tena', i.e. by him, implies Brahma. They are meant to worship the Ātman. The Vedic injunctions are an expression of that. The word 'purā', i.e. before, shows it is not spoken by the Lord now, but is an ancient knowledge.