From Bhagavad-gita As It Is, Introduction
THE ENGLISH WORD religion is a little different from sanatana-dharma. Religion conveys the idea of faith, and faith may change. One may have faith in a particular process, and he may change this faith and adopt another, but sanatana-dharma refers to that activity which cannot be changed. For instance, liquidity cannot be taken from water, nor can heat be taken from fire. Similarly, the eternal function of the eternal living entity cannot be taken from the living entity.
Sanatana-dharma is eternally integral with the living entity. When we speak of sanatana-dharma, therefore, we must take it for granted on the authority of Sripada Ramanujacarya that it has neither beginning nor end. That which has neither end nor beginning must not be sectarian, for it cannot be limited by any boundaries.
Those belonging to some sectarian faith will wrongly consider that sanatana-dharma is also sectarian, but if we go deeply into the matter and consider it in the light of modern science, it is possible for us to see thatsanatana-dharma is the business of all the people of the world nay, of all the living entities of the universe.