SB 4.12.36
yad bhrajamanam sva-rucaiva sarvato
lokas trayo hy anu vibhrajanta ete
yan navrajañ jantusu ye ’nanugraha
vrajanti bhadrani caranti ye ’nisam
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The self-effulgent Vaikuntha planets, by whose illumination alone all the illuminating planets within this material world give off reflected light, cannot be reached by those who are not merciful to other living entities. Only persons who constantly engage in welfare activities for other living entities can reach the Vaikuntha planets.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Here is a description of two aspects of the Vaikuntha planets. The first is that in the Vaikuntha sky there is no need of the sun and moon. This is confirmed by the Upanisads as well as Bhagavad-gita (na tad bhasayate suryo na sasanko na pavakah). In the spiritual world the Vaikunthalokas are themselves illuminated; there is therefore no need of sun, moon or electric light. It is in fact the illumination of the Vaikunthalokas which is reflected in the material sky. Only by this reflection are the suns in the material universes illuminated; after the illumination of the sun, all the stars and moons are illuminated. In other words, all the luminaries in the material sky borrow illumination from Vaikunthaloka. From this material world, however, people can be transferred to the Vaikunthaloka, if they incessantly engage in welfare activities for all other living entities. Such incessant welfare activities can really be performed only in Krsna consciousness. There is no philanthropic work within this material world but Krsna consciousness that can engage a person twenty-four hours a day.
A Krsna conscious being is always engaged in planning how to take all of suffering humanity back home, back to Godhead. Even if one is not successful in reclaiming all the fallen souls back to Godhead, still, because he is Krsna conscious, his path to Vaikunthaloka is open. He personally becomes qualified to enter the Vaikunthalokas, and if anyone follows such a devotee, he also enters into Vaikunthaloka. Others, who engage in envious activities, are known as karmis. Karmis are envious of one another. Simply for sense gratification, they can kill thousands of innocent animals. Jñanis are not as sinful as karmis, but they do not try to reclaim others back to Godhead. They perform austerities for their own liberation. Yogis are also engaged in self-aggrandizement by trying to attain mystic powers. But devotees, Vaisnavas, who are servants of the Lord, come forward in the actual field of work in Krsna consciousness to reclaim fallen souls. Only Krsna conscious persons are eligible to enter into the spiritual world. That is clearly stated in this verse and is confirmed in Bhagavad-gita, wherein the Lord says that there is no one dearer to Him than those who preach the gospel of Bhagavad-gita to the world.