Advaita crystallizes the Upanishadic great sayings, tat tvam asi (Chandogya Upanishad 6.8.7) and aham brahmasmi (Brihadaranyaka 1.4.10), into a system. Adi Shankara (traditionally c. 788-820) wrote the Brahma Sutra Bhashya and commentaries on the principal Upanishads, arguing the world is maya, appearance superimposed on the real the way a rope is mistaken for a snake, and liberation (moksha) is knowledge (jnana), not ritual. Gaudapada's Mandukya Karika is the tradition's earlier philosophical spine.
Global spirituality absorbed Advaita as 'we are all one', a greeting-card monism Shankara would have failed: his system demands discrimination between the real and the apparent (viveka) and years of hearing, reflection, and meditation under a teacher. Neo-Advaita's claim that no practice is needed is itself, in classical terms, a position held by the ego.
Advaita's core move is epistemic: you do not need to create unity, only to stop misreading difference. Liberation adds nothing to the world; it subtracts the error, the rope was never a snake. It is the philosophical tradition that treats enlightenment as a correction, not an acquisition.
If only Brahman is real, who is having this thought?