The Ancient Period is characterised by the worship of Shiva (Pashupati) during the Harappan times; the composition of the early Rig Vedic hymns; the Epic Period during which the Ramayana and Mahabharata were composed; period of Sage Vishvamitra, in whose reign a majority of the Vedic hymns were composed; the Yajur and Atharva Vedas were composed around 2400 BC; the Bhagwad Gita was compiled between BC 500-200; Nyaya, Sankhya, and Brahma Sutras were recorded, which later gave birth to six popular schools of Hindu philosophy and final versions of Puranas, Tantras and other sectarian literature were developed (200-750 AD). Development of six popular schools of religious thought, establishment of Shankara's Advaita Vedanta and the decline of Buddhism are the main landmarks of this period (750-1000 AD).
The Medieval Period saw the rise of devotional movements led by Ramanuja, Ramananda, Tukaram, Guru Nanak, Surdas, Chaitanya, Mirabai, Tulsi Das and many other saints.
The Bhakti movement is much older than the Sufi movements. It philosophical concept had been fully enunciated in the Upanishads and subsequently, the Bhagavad Gita emphasized love and devotion as pathways to God.
In the sixth century A.D., the Shagavata Purana placed the concept of bhakti on a very high pedestal. During the post-Bhagavata phase passionate love and devotion to one personal God became a characteristic feature of the Indian religious thought.
But the concept of bhakti was placed on a firmer ground in South India, when Shankaracharya revived the philosophy of Advaita or Vedanta. After Shankara, twelve Tamil Vaishnava saints collectively known as Alvars made the concept of bhakti more popular.
The Alvars were followed by the Vaishnava acharyas who gave the Bhakti cult a metaphysical foundation. According to this school of thought