Even after so many centuries have passed since Sri, Sankara appeared; yet it is very hard to find his true teachings understood anywhere in the world, because so few could rise to his level.
Sage Sri, Sankara gave religion and scholasticism and yoga no less than philosophy, to the world. He was great enough to be able to do so. His commentary on Mandukya is pure philosophy, but many of his other books are presented from a religious standpoint to help those who cannot rise up to philosophy.
Ultimate truth or Brahman can be established and conformed mentally by soul-centric reason independent of scriptures. It is possible also to show the unreality of the objective universe even from well directed pure reasoning.
The scripture are for the ordinary man who are immersed in practical life within the practical world, who finds security in the idea of causality and thus in the idea of God.
Those of superior intelligence, have no need of this idea of divine causality, and can therefore dispense with Vedas and arrive at the truth of Non-Dualism by pure reason.
People, who did not recognize the authority of the scriptures, will be able to prove the truth of non-dualistic by means of reason alone. Ultimate truth does not need the support of any Scripture or Revelation like the Veda. It is based, not upon the varying theological fancies, which are as numerous as the sands of the sea, but upon reason, the common heritage of all mankind, irrespective of colour or creed or clime. :--FORMLESS PATH