Krishna says, "'That which is to be known' is the Sustainer, Destroyer and Creator of beings; indivisible and yet existing as if divided in living beings (13.17). The Light of lights; Beyond the darkness. He is knowledge, the object and the goal of knowledge. He dwells in the hearts of all (13.18). Realising this, My devotee attains My Being" (13.19).
"He dwells in the hearts of all" was dealt with earlier when Krishna said that he who has perfected in yoga finds the wisdom in the self (3.38). 𝙎𝙝𝙧𝙖𝙙𝙙𝙝𝙖-𝙫𝙖𝙖𝙣 (one with 𝙎𝙝𝙧𝙖𝙙𝙙𝙝𝙖 ) and 𝙅𝙞𝙩𝙚𝙣𝙙𝙧𝙞𝙮𝙖 (one who conquered senses) obtain wisdom leading to 𝙥𝙖𝙧𝙖𝙢𝙖-𝙨𝙝𝙖𝙣𝙩𝙞 (supreme peace) (3.39). The ignorant, devoid of 𝙎𝙝𝙧𝙖𝙙𝙙𝙝𝙖 is ruined and there is no happiness for him in this world or another (3.40).
HE is indivisible and yet exists as if divided in the living beings, the inability to comprehend this at the existential level is the source of misery. It is like the proverbial elephant and five blind men who could comprehend only a part of the elephant leading to differences and disputes.
The current scientific understanding also points to this indivisibility in terms of quantum fields permeating the entire universe. Matter or particles are nothing but excitement in a quantum field.
One easy way to inculcate this 'Indivisibility' or oneness is to start understanding others' points of view without making any assumptions and by asking questions when in doubt. This comes naturally when one becomes a mother or father to understand the needs of a baby; when one moves to higher positions in the workplace. The key is to expand this learning in every aspect of life.
Uniting opposites as One is the key to attaining 𝙋𝙖𝙧𝙖𝙢𝙖𝙩𝙢𝙖 as He is both. This is nothing but 'no mind' where the mind stops dividing.