There is reference to the word sanātanam in the tenth verse of the Seventh Chapter, in which the Lord says that He is the eternal fountainhead of everything and is therefore sanātanam. The fountainhead of everything is described in the Upaniṣads as the complete whole. All emanations of the fountainhead are also complete in themselves, but although many complete units emanate from the complete sanātana fountainhead, the sanātana head does not diminish in quality or quantity.* That is because the nature of sanātana is unchangeable. Anything that changes under the influence of time and circumstances is not sanātana. Therefore anything that changes whatsoever in form or quality cannot be accepted as sanātana. To give a material example, the sun has been disseminating its rays for hundreds and millions of years, and yet although it is a materially created object, its form and rays are still unchanged. Therefore, that which is never created cannot change in formation and quality, even though He is the seedling source of everything.
The Lord claims to be the father of all species of life. He claims that all living beings—regardless of what they are—are part and parcel of Him. Consequently, the Bhagavad-gītā is meant for all of them. In the Gītā there is information of this sanātana nature of the Supreme Lord. There is also information of His abode, which is far beyond the material sky, and of the sanātana nature of the living beings.
Lord Kṛṣṇa, in the Bhagavad-gītā, also informs us that this material world is full of miseries in the shape of birth, old age, disease and death. Even in the topmost planet of the material universe, Brahmaloka, these miseries are present. Only in His own abode is there a total absence of misery. In that abode there is no need of light from sun, moon or fire. The planets are self-luminous. Life there is perpetual and full of knowledge and bliss. That is what is known as sanātana-dharma. It is therefore natural to conclude that the living entities must return home, back to Godhead, to enjoy life in the sanātana-dhāma with the sanātana-puruṣa, or the puruṣottama, Lord Śrī Kṛṣṇa. They must not remain to rot in this miserable land of material existence. There is no happiness in the material sphere—even in Brahmaloka—so plans and activities for elevation to higher planets within the material universe are carried out by those who are less intelligent. Less intelligent men also take shelter of demigods and only derive benefits which endure for a limited period. Thus their religious principles and the benefits derived therefrom are only temporary. The intelligent man, however, abandons all engagements in the name of religion and takes shelter of the Supreme Personality of Godhead and thus receives absolute protection from the Almighty Father. Sanātana-dharma is therefore the process of bhakti-yoga, by which one can come to know the sanātana Lord and His sanātana abode. By this process only can one return to the spiritual universe, the sanātana-dhāma, to take part in the sanātana enjoyment prevailing there.