Hinduism is polytheistic (belief in more than one God) in that it grants many names and forms to divine force, but it is ultimately monotheistic ( belief of one God) as well, in that all forms of divinity can be reduced to one: Devi ("The Goddess").  True, there are  some Gods of whom have more power than the Goddess. But without her, they would have no power at all, or even a form.
  For it is Devi, the Hindus say, who gives birth to all force and form, who creates separation out of unity, who is the energy without which all would still be chaos.

  All Goddesses are Devi, the one Goddess; all the myths told of black Kali, or Golden Parvati or Guari, of the fierce Durga are myths of Devi.

 The civilization of the Indus Valley in the 2nd millennium B.C. is known to have been centered on a religion of the Goddess.  But generations of invasions and warfare almost obliterated the early worship.  Still, followers of the Goddess retained their beliefs and rituals, which erupted in later Indian history in the Shaktic and Tantric movements.  Almost 3,000 years passed between the heyday of Devi's worship and the Indian Middle Ages, but when she reemerged it was not only as the people's deity, the popular recipient of their devotions, but as the philosophic basis of the perceived universe.