"In this material world we find that everything is temporary. It comes into being, stays for some time, produces some by-products, dwindles and then vanishes. That is the law of the material world, whether we use as an example this body, or a piece of fruit or anything. But beyond this temporary world there is another world of which we have information. This world consists of another nature, which is sanatana, eternal. Jiva is also described as sanatana, eternal, and the Lord is also described as sanatana in the Eleventh Chapter. We have an intimate relationship with the Lord, and because we are all qualitatively one--the sanatana-dhama, or sky, the sanatana Supreme Personality and the sanatana living entities--the whole purpose of Bhagavad-gita is to revive our sanatana occupation, or sanatana-dharma, which is the eternal occupation of the living entity. We are temporarily engaged in different activities, but all of these activities can be purified when we give up all these temporary activities and take up the activities which are prescribed by the Supreme Lord. That is called our pure life.
The Supreme Lord and His transcendental abode are both sanatana, as are the living entities, and the combined association of the Supreme Lord and the living entities in the sanatana abode is the perfection of human life. The Lord is very kind to the living entities because they are His sons. Lord Krsna declares in Bhagavad-gita, sarva-yonisu... aham bija-pradah pita: "I am the father of all." Of course there are all types of living entities according to their various karmas, but here the Lord claims that He is the father of all of them. Therefore the Lord descends to reclaim all of these fallen, conditioned souls to call them back to the sanatana eternal sky so that the sanatana living entities may regain their eternal sanatana positions in eternal association with the Lord. The Lord comes Himself in different incarnations, or He sends His confidential servants as sons or His associates or acaryas to reclaim the conditioned souls.
Therefore, sanatana-dharma does not refer to any sectarian process of religion. It is the eternal function of the eternal living entities in relationship with the eternal Supreme Lord. Sanatana-dharma refers, as stated previously, to the eternal occupation of the living entity. Ramanujacarya has explained the word sanatana as "that which has neither beginning nor end," so when we speak of sanatana-dharma, we must take it for granted on the authority of Sri Ramanujacarya that it has neither beginning nor end.
The English world 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 Sri 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 that sanatana-dharma is the business of all the people of the world--nay, of all the living entities of the universe.
Non-sanatana religious faith may have some beginning in the annals of human history, but there is no beginning to the history of sanatana-dharma because it remains eternally with the living entities. Insofar as the living entities are concerned, the authoritative sastras state that the living entity has neither birth nor death. In the Gita it is stated that the living entity is never born, and he never dies. He is eternal and indestructible, and he continues to live after the destruction of his temporary material body. In reference to the concept of sanatana-dharma, we must try to understand the concept of religion from the Sanskrit root meaning of the word. Dharma refers to that which is constantly existing with a particular object. We conclude that there is heat and light along with the fire; without heat and light, there is no meaning to the word fire. Similarly, we must discover the essential part of the living being, that part which is his constant companion. That constant companion is his eternal quality, and that eternal quality is his eternal religion."
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.