The Devi Mahatmya, seven hundred verses forming chapters eighty-one to ninety-three of the Markandeya Purana, is the founding text of goddess worship as a self-standing tradition rather than a department of someone else's. Its claim is not that the Goddess is powerful, which nobody disputed, but that she is supreme: prior to the gods, the source of their power, and not derived from a husband.
Its central episode makes the argument structurally. The buffalo demon Mahisha cannot be killed by any god, and the gods, comprehensively defeated, gather and release their accumulated energies, which combine into a single blazing female form. Each god gives her his own weapon: Shiva's trident, Vishnu's discus, Indra's thunderbolt. She rides a lion, she laughs, and she kills in an afternoon the enemy that had beaten all of them together. The gods are not her superiors in this account. They are her components.
Dating is unusually tractable for a Sanskrit text. The Devi Mahatmya is generally placed between about 400 and 600, added to a Markandeya Purana whose base Rocher put around the third century. Coburn and Mackenzie Brown put the addition in the fifth or sixth century, and Doniger O'Flaherty gives roughly 550. There is a hard limit: the Dadhimati Mata inscription of 608 quotes portions of the text, so it cannot be later than the seventh century. Inscriptional quotation of a religious text is the strongest kind of terminus available, and it is worth more than any stylistic estimate.
Whether Durga, Kali, Parvati and the rest are distinct goddesses or aspects of one is exactly what Shakta theology asserts and non-Shakta traditions do not concede. The Devi Mahatmya takes the strong position: they are one, and the gods are her expressions rather than the reverse.