Arjuna inquired: O Purushottama (Supreme Person), what is Brahman? What is the self? What is karma? What is action? What is this material manifestation? Who are the demigods? Who is the object of sacrifice and how does He dwell within the body? O Madhusudana, how can You be known by those who are self-controlled at the time of death?
Bhagavad Gita - Chapter 8 - Taraka Brahma Yoga - The Yoga of the Supreme
Bhagavan Shri Krishna replied: It is said that Brahman is the imperishable Supreme, and the atma is the original spiritual nature of the living being. Karma is that which produces birth, duration of life and death.
This material manifestation is defined as constantly changing and this material universe is the cosmic form of the Supreme Person. I am the object of all sacrifices, situated in all living beings.
At the time of death, one who leaves the body specifically remembering Me attains My nature – of this there is no doubt.
O son of Kunti, whatever state of being one remembers at the time of death, one certainly attains to that nature.
Therefore, always remember Me and fight! Surrender your mind and intelligence to Me and you will surely attain Me.
O Partha, one who practices yoga and focuses his mind, without straying from the path, and meditates upon the Divine Supreme Person, certainly achieves Him.
One should meditate upon the Supreme Person who is allknowing, without beginning, the Supreme Controller, who is smaller than the atomic particle, yet who is the support of all things in the universe, whose form is inconceivable, who is radiant like the sun, and who is beyond material nature.
At the time of death, one who remembers Him with a steady mind by drawing the life-airs between the eyebrows by dint of yogic power certainly reaches the Divine Supreme Person.
Great sages in the renounced order of life and scholars of the Vedas accept the vow of celibacy and utter om to enter into Brahman. I shall now explain this process to you.