“From such Scriptural passages as, ‘There is no multiplicity in Ātman’, ‘Indra through Māyā’ etc., we know that the Ātman, though ever unborn, verily appears to have become many {only) through Māyā.” —Māṇḍūkya Upaniṣad Kārikā, 3.24

Śaṅkarācārya’s Commentary:
It may be asked how the changelessness (Ajati) of Atman is the final conclusion of the Sruti. In reply it is said that if creation were real, then the existence of the variety of objects would be absolutely real. Consequently there ought not to be Scriptural texts implying their unreality. But there are such Scriptural texts as, “In this {Atman) there is no multiplicity,” etc., which negate the existence of duality. Therefore creation (imaginary) has been imagined in order to help the understanding of the non-duality of Atman. It is like the story of Prana. And this is further borne out by the use of the word, “Maya,” denoting unreality (in connection with creation) in such Scriptural texts as “Indra through Maya assumed diverse forms”.