Brahman and Ishvara — The Absolute and the Personal God
Advaita Vedanta distinguishes between Nirguna Brahman (the absolute without qualities) and Saguna Brahman/Ishvara (the personal God with qualities) — both pointing to the same reality at different levels of understanding.
Shankara's Advaita distinguishes two levels of truth and correspondingly two ways of speaking about ultimate reality: Vyavaharika (conventional/relative truth — the level of everyday experience) and Paramarthika (absolute truth — the level of non-dual awareness). At the conventional level, Saguna Brahman (Brahman with qualities — Ishvara, the personal God) is real and can be worshipped, prayed to, and related with. At the absolute level, Nirguna Brahman (Brahman without qualities — the formless absolute) is the only reality. The personal God (Ishvara) is Brahman seen through the medium of Maya — real as an appearance, but not the final truth. This creates the possibility of both devotional worship (bhakti) and non-dual knowledge (jnana) in Advaita: the bhakta approaches Brahman through the personal God; the jnani approaches directly. Both paths lead to the same recognition — though Jnana Yoga is considered the direct path in the Advaita hierarchy.
Ishvara is Brahman seen through Maya; Brahman is Ishvara recognized as the absolute.
Philosopher Wilfried Cantwell Smith's distinction between 'faith' (an inner quality of personal orientation toward the transcendent) and 'belief' (intellectual assent to propositions about God) parallels the Advaita distinction between bhakti (devotional orientation toward Ishvara) and jnana (direct non-conceptual recognition of Nirguna Brahman).
Brahmasutra Bhashya