Brahman in Hinduism is that cosmic entity, primordial cosmic energy, big bang explosion of which resulted in creation of present cosmos. Upanishads (independent spiritual treatises) part of Hinduism detailed in absolute clarity everything relating to origin of cosmos.
Imagine a situation in cosmos when old cosmos is dissolving. This dissolution of cosmos (called Pralaya in Hinduism) occurs at a very fast pace as if the whole massive cosmos is vanishing inside a big black hole and it finally does, reduces to size of half a thumb.
This half a thumb, the combined power of all souls’ atmans in cosmos in their pure form is what we called God Almighty in all its glory (Brahman in Hinduism).
This primordial cosmic energy, cluster of pure souls’ atmans unable to, contain itself for long in its prime pure state again explodes with a big bang resulting in formation of a new cosmos and the present cosmos came into existence.
All souls’ atmans including Brahman exist from times immemorial yet; God Almighty (Brahman) is the creator of present cosmos.
Relationship of Atman with Brahman Soul with God
If an individual grain of sand is an independent soul atman, the whole mound God Almighty Brahman. If an individual piece of puzzle is an independent soul atman, the completed puzzle God Almighty, Brahman.
In cosmic reality God Almighty existed in all living beings in miniscule form as their soul atman.
We are primarily a spirit, a spiritual traveler and not the manifest physical human form. We must learn to identify self from things physical. If we can demolish our ego in totality, we can realize this cosmic truth of life that Lord Krishna truly existed in our heart as our soul atman.
Allegorically Lord Krishna is the spirit, soul atman existing in our heart. Indulging in spirituality detailed in Bhagavad Gita if we can reach Lord Krishna within, we shall also reach God Almighty, Brahman… finally merging with him forever.