In Hindu monotheism, the concept of God varies from one sect to another. Hinduism (by its nature as a regional rather than a doctrinal religious category) is not exclusively monotheistic, and has been described as spanning a wide range of henotheism, monotheism, polytheism, panentheism, pantheism, pandeism, monism, atheism and nontheism etc.
The philosophical system of Advaita or non-dualism as it developed in the Vedanta school of Hindu philosophy, especially as set out in the Upanishads and popularised by Adi Shankara in the 9th century, would become the basis of mainstream Hinduism as it developed in the medieval period. This non-dualism postulates the identity of the Self or Atman with the Whole or Brahma, and can be described as monism or pantheism.
Forms of explicit monotheism find mention in the canonical Bhagavad Gita. Explicit monotheism in the form of emotional or ecstatic devotion (bhakti) to a single external and personal deity (in the form of Shiva or Vishnu) became popular in South India in the early medieval period. Ecstatic devotion to Krishna, a form of Vishnu, gained popularity throughout India during the Middle Ages and gave rise to schools of Vaishnavism. Ecstatic devotion to Goddess Durga became popular in some parts of India in the later medieval and early modern ages. Vaishnavism, particularly Krishnaism, Shaktism and some forms of Shaivism remain the most explicit forms of monotheistic worship of a personal God within Hinduism. Othe . . . more