Brahman & Atman represent the Paradox of Oneness. They represent the duality of being at One with God and being at One with the Self. It is the paradox of being a part of God and apart from God.
Every paradox is an effect of the dual nature of reality. Duality allows choice and choice allows two contrasting or opposing perspectives of reality.
Brahman is the ultimate or absolute reality of Singularity, where all is Oneness.
Atman is the personal reality of the ultimate Self or Soul. The Id, the Ego & the Entity are a spiritual triality, or triune reality, which is the ultimate test or trial for humankind.
Duality allows a choice of polarity. Polarity is a choice of direction, which is seen to have either a positive or a negative outcome. I can travel back to the singular Oneness of Brahman or I can take the path of my Atman or spiritual Soul.
In this three dimensional reality, the Soul has three aspects of a physical body, a mental mind and an emotional feeling. The Soul is the Self in its purest form.
Atman is therefore, the purest expression of the oneness or unity of the Soul/Self.
Brahman is the realisation that everything is manifest from the One Divine Source of All That Is.
Following the destiny of my Atman is an individual, unique & exclusively personal experience. Each Self individually experiences the purity & Oneness of the Divine Soul as its own personal destiny.
Following the path or way of Brahman is a collective, standardised, inclusively group experience, often within a Religion or a Cult.
Atman is the pursuit of personal spirituality, whereas Brahman is the pursuit of religious spirituality.
Atman is an exclusive connection to Source, whereas Brahman is an inclusive attachment to Source.
In absolute reality, Brahman & Atman are the same Source Entity, seen from contrasting polarities or perspectives. Source is both collectively, the Soul and individually, the Self. Brahman is the Soul Entity, whereas Atman is the Soul/Self.
Brahman is the Source, Atman is the Destiny.
Brahman is the Alpha and Atman is the Omega.