The authority of the Upanishads and other Vedic literature were revived by Shankara and employed as weapons to fight the Buddhist doctrine. Interpreting the Vedas to draw a particular conclusion, Shankara established the doctrine of non-dualism, adwaita-vedanta, stating that all living entities were on an equal level with God. He prominently stressed those texts which afforded an answer to the rationalistic atheism of the Buddhists, yet the teachings of Shankara were not wholly theistic, and thus a further unveiling of the ultimate reality was destined. That destiny was fulfilled through Shri Ramanujacharya"
More about Sripad Ramanujacarya in the History of Vaishnavas here:
http://gaudiyahistory.com/ramanujacarya/
Also devotional videos/dramas enacting the Life, Pastimes, Mission of Sripad Ramanujacarya
http://www.iskcondesiretree.net/video/video/search?q=Ramanujacharya