Margaret Stutley, Dictionary of Hinduism: Its Mythology, Folklore and Development 1500 BC - AD 1500

 “Shankara’s views were gradually accepted, possibly because he presented brahman both as the cosmic principle and as a personal god (isvara), which added emphasis to the teaching of the later Upanisads and to that of Patañjali. Advaita Vedanta thus reinforced the teaching of the Bhagavadgitä and the concept of liberation (mukti) by grace (prasäda), faith (sraddha), and devotion (bhakti). It succeeded in reviving the ancient belief in the affinity of mankind with the world of nature. From being merely one of the darsanas, the Vedanta became an element that permeated all Hindu cults and dissolved sectarian distinctions. It gave to the Supreme Essence (paramätman), Vishnu and Shiva the common, all-inclusive designation, ‘Isvara’.”