Lord Krishna assured Arjuna that a person who is sincerely committed to the constant practice of meditation of Lord, with single minded devotion, can reach Him easily. Here we should remember that when Lord refers to Himself in first person singular using term ‘I” and “Me”, He is talking as Paramathma, Akshara Brahman. He also explains to Arjuna why attaining The Lord is the most desirable goal for one. People of the world seeking permanent peace, security and happiness have either Paramathma or world, as their goal. Those virtuous people who have Paramathma as their goal and meditate on Him sincerely with single-minded devotion praying for His Grace and Grace only, reach Him after death, totally freed from samsara and the cycle of birth and do not return to earthly existence at all. They have attained Moksha, the ultimate goal. On the other hand those seeking worldly goals, even if they go to heavenly lokas after death due to their punya, return to earth and the life of sorrow and samsara at the exhaustion of punya and continue to struggle. This cycle of birth and death continues until they change their goal and seek Lord and Lord only in their earthly life and attain Him through meditation and single-minded devotion. This is true even of the virtuous people engaged in ritualistic worship and Sakama upasana as well, who go to any one of the higher lokas other than Brahma loka, after death, be it bhuvar loka, suvar loka, mahar loka, jana loka, tapo loka, or satya loka, declares Lord Krishna. In these lokas they enjoy, no doubt, the joys and comforts of the higher lokas, free from the miseries of disease, death and old age, but it is only as long as their stock of punya allows them and get shunted back to earth to continue in the cycle of birth and death in struggle and samsara.
Attaining Paramathma is obtaining the Ikya Jnanam through which one is unshakeably established in the firm knowledge of the identity of one’s Athma with Paramathma. So only those who have got the Brahma Jnanam whether on earth or in the Brahma loka, are freed totally from the cycle of birth and death. Only Paramathma who is eternal is not subject to time. To drive home this point firmly Lord points out in verse 17 that even Brahmaji, as we shall refer to Lord Brahma to distinguish Him from Brahman, is subject to time.
Those who know the day of Brahma, which is of a duration of one thousand chathur Yugas, and the night, which is also of one thousand chathur Yugas’ duration, they know day and night. (8-17)
One chathur yuga consists of four yugas. They are Krita yuga, Threta yuga, Dvapara yuga and Kali yuga. Together they run for 4,320,000 years. Brahmaji’s one full day consists of daytime of 1000 chathur yugas and night-time of 1000 chathur yugas, i.e. in total 2000 chathur yugas. 365 such days make Brahmaji’s one year. Brahmaji’s lifetime consists of one hundred such years. So His life runs much much longer but still it is also limited. When night falls for Brahmaji, all the lokas except Brahma loka dissolve into Maya, in what is called pralaya and manifest again when Brahmaji’s day starts which is called creation. At the end of Brahma’s lifetime all the lokas including Brahma loka are dissolved into Maya and later spring back into manifestation as a new creation. In the case of jivas this cycle of manifestation and unmanifestation goes on eternally until the jivas exhausting their karma totally through Athma Jnanam attain the Lord. So in verses 18 & 19 Lord Krishna states:
From the unmanifested all the manifested (worlds) emerge at the coming of the “day”; at the coming of the “night” they dissolve merging into that itself which is called the unmanifested. (8-18)
This same multitude of beings, born again and again, is dissolved, helplessly, O Arjuna, (into the unmanifested) at the coming of the "night", and comes forth at the coming of the "day"! (8-19)
Thus the worlds and beings go in a cyclic form of manifestation and unmanifestation. When Brahmaji’s life time ends, all the worlds without exception i.e. including Brahma loka also dissolve and Akshara Brahman is the only survivor at the end of Brahmaji’s life which is called Mahapralaya. If we designate Maya as unmanifest then it is lower unmanifest and Paramathma, the Akshara Brahman who is beyond Maya is higher Unmanifest and the lower unmanifest merges with the higher Unmanifest at the time of Maha pralaya. That is why Lord Krishna states in the next verse:
But higher than this unmanifested, there exists another Unmanifested Eternal Being, who does not perish when all beings perish. (8-20)
Reaching This Unmanifested Eternal Being, who is also called Paramathma and Akshara Brahman, a Jiva is Liberated from the cycle of birth and death. Jnani attains this Liberation as a videhamukthi involving no travel after death; nishkama upasaka travels to Brahma loka and attains Liberation there as krama mukthi and sakama upasaka and other karmis travel to other lokas only to return to earth and its pleasures, pains and sorrows. The other lokas are the lokas between bhu loka and Brahma loka and all these intermediary lokas are referred by a general name swarga loka. The path nishkama upasaka takes to Brahma loka after death is the path of no-return and is called Sukla ghathi and the path sakama upasaka and other virtuous people, including karmi, take to swarga loka after death is the path of return and is called Krishna gathi. Sukla gathi, also called bright path by the Lord, the path of no return, is guided by five devathas. They are agni devatha, jyothir devatha; ahar devatha (devata presiding over day time); sukḷa pakṣha devatha; uttarayaṇa devatha. Krishna gathi, also called dark path by the Lord, the path of return, has four devathas as guides. They are dhumaḥ devatha (devata presiding over smoke); ratri devatha (devata presiding over night time); kṛiṣhṇa pakṣha devatha and dakṣhiṇayana devatha. These two paths, the bright path and the dark path are also considered to be eternal. As Lord Krishna concludes this topic, He glorifies Nishkama Saguna Brahma upasaka, calling him a Yogi. He says Yogi knowing the limitations of various rituals, study of Vedas and charitable acts, pursues steadfastly Nishkama Saguna Brahma upasana and attains Moksha, through krama mukthi and He also advises Arjuna to become a Yogi and attain the Supreme Lord.