The Lord comes Himself in different incarnations, or He sends His confidential servants as sons or His associates or ācāryas to reclaim the conditioned souls. Therefore, sanātana-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. Sanātana-dharma refers, as stated previously, to the eternal occupation of the living entity. Śrīpāda Rāmānujācārya has explained the word sanātana as "that which has neither beginning nor end," so when we speak of sanātana-dharma, we must take it for granted on the authority of Śrīpāda Rāmānujācārya that it has neither beginning nor end.
(Bhagavad-Gita--introduction).

The English world religion is a little different from sanātana-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 sanātana-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. Sanātana-dharma is eternally integral with the living entity. When we speak ofsanātana-dharma, therefore, we must take it for granted on the authority of Śrīpāda Rāmānujācārya 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.
(Bhagavad-Gita--introduction).

In the Bhagavad-gītā the Lord and the living beings are both described as sanātana, or eternal, and the Lord's abode, far beyond the material sky, is also described as sanātana. The living being is invited to live in the sanātana existence of the Lord, and the process which can help a living being to approach the Lord's abode, where the liberated activity of the soul is exhibited, is called sanātana-dharma. One cannot, however, reach the eternal abode of the Lord without being free from the misconception of material identification, and the Bhagavad-gītā gives us the clue how to achieve this stage of perfection.
(Srimad Bhagavatam----1:15:27----purport).

In the process of glorifying the Lord there should be no restriction of time and space. This is called sanātana-dharma or bhāgavata-dharma. Sanātana means eternal, always and everywhere. Bhāgavata means pertaining to Bhagavān, the Lord. The Lord is the master of all time and all space, and therefore the Lord's holy name must be heard, glorified and remembered everywhere in the world. That will bring about the desired peace and prosperity so eagerly awaited by the people of the world.
(Srimad Bhagavatam----2:2:36----purport).

This system is sanātana—it comes from time immemorial, and it will continue in the same way. There is no power in the world which can stop it. Therefore, since thissanātana-dharma system is eternal, one can elevate himself to the highest standard of spiritual life by following the Vedic principles.
(Srimad Bhagavatam----4:2:31----purport).

All the activities of the conditioned soul within this material world are perpetually taking place in different types of bodies, but when the living entity acquires developed consciousness, he should try to rectify his situation and again become a member of the spiritual world. The process by which one can return home, back to Godhead, is bhakti-yoga, sometimes called sanātana-dharma. Instead of accepting a temporary occupational duty based on the material body, one should take to the process of sanātana-dharma, or bhakti-yoga, so that he can put an end to this perpetual bondage in material bodies and return home, back to Godhead.
(Srimad Bhagavatam----4:28:61----purport).

The entire Kali-yuga is full of faults. It is like an unlimited ocean of faults. But the Kṛṣṇa consciousness movement is very authorized. Therefore, following in the footsteps of Śrī Caitanya Mahāprabhu, who five hundred years ago inaugurated the movement of saṅkīrtana, kṛṣṇa-kīrtana, we are trying to introduce this movement, according to superior orders, all over the world. Now, if the inaugurators of this movement strictly follow the regulative principles and spread this movement for the benefit of all human society, they will certainly usher in a new way of life by reestablishing sanātana-dharma, the eternal occupational duties of humanity. The eternal occupational duty of the human being is to serve Kṛṣṇa. Jīvera 'svarūpa' haya-kṛṣṇera 'nitya-dāsa.' (CC-2:20:108). This is the purport of sanātana-dharma. Sanātana means nitya, or "eternal," and kṛṣṇa-dāsa means "servant of Kṛṣṇa." The eternal occupational duty of the human being is to serve Kṛṣṇa. This is the sum and substance of the Kṛṣṇa consciousness movement.
(Srimad Bhagavatam----8:14:4----purport).