Hinduism scriptures primarily Bhagavad Gita and Upanishads (independent spiritual treatises) detailed definition of atman and Brahman very distinctively.
In Hinduism atman means Prana energy, the spirit that manifests human body to work out its karma. The human form is a medium for atman within to reach state of absolute purity. Of its own no atman can purify itself and needs a medium, a body to cleanse self of dross impurities within.
Atman called soul is the basic essence of life on mother earth. In absence of atmans there would not be any life on mother earth.
Only when Brahman, God Almighty exploded self with a big bang the present cosmos came into existence. But this resulted in pure atmans scattering all over cosmos at unimaginable speeds and in the melee they gathered impurities.
No sooner life supporting planets like mother earth evolved, these hurtling atmans immediately manifested bodies and life came into existence on mother earth.
All atmans were not distinct from Brahman. If an individual grain of sand is an independent atman the whole mound Brahman, Parmatman. If an individual piece of puzzle is an independent atman, the completed puzzle Brahman, God Almighty.
Spiritually God Almighty existed in all living beings including human beings in miniscule form as our atman within.
To understand everything relating to atman, Brahman our indulgence in spirituality as detailed in Bhagavad Gita is the only route.
Hinduism – The Upanishads : With the last component of the Vedas, the philosophically oriented and esoteric texts known as the Upanishads (traditionally “sitting near a teacher” but originally understood as “connection” or “equivalence”), Vedic ritualism and the doctrine of the interconnectedness of separate phenomena were superseded …