Srila Vyasadeva wrote down the Veda en Purana. He then divided the Veda in 4 parts. He wrote down Mahabharata. Then he wrote Vedantasutra (Brahma Sutra). And the commentary on Vedantasutra is the Srimad Bhagavatam. To furthur explain Srimad Bhagavatam, Srila Jiva Gosvami wrote the Sad-Sandarbas.
In Bhagavad Gita As It Is (is to be found in Mahabharata Chapters 25-42 of the Bhisma Parva), the following conclusion can be made:
Lesson 18 - The Final Instruction of Lord Krsna:
“Abandon all varieties of religion and just surrender unto Me. I shall deliver you from all sinful reactions. Do not fear.” (Chapter 18)
Chapter 18: Conclusion—Perfection of Renunciation
“Actually, the Bhagavad Gita is finished in 17 chapters. The 18th chapter is a supplementary summarization of the topics discussed before…” (SP’s purport to 18.1)
Krsna explains: The meaning of renunciation and the effects of the modes of nature on human consciousness and activity, Brahman realization, The glories of the Bhagavad Gita, and the ultimate conclusion of the Gita—the highest path of religion is absolute, unconditional loving surrender unto Lord Krsna, which frees one from all sins, brings one to complete enlightenment, and enables one to return to Krsna's eternal spiritual abode.
(68-71):
Arjuna is firmly fixed—”all my misgivings and misunderstandings have now gone by Your mercy. I have decided to fight because YOU (Krsna) desire it.”
Difference between bhakti and karma (SP):
Sanjaya after narrating this conversation to Dhrtarastra, ecstatically thinks of the universal form of Krsna and predicts victory for Arjuna, the supreme archer, for he is surrendered to Krsna, the master of all mystics.
This is his reply to Dhrtarastra’s question (1.1), which is the opening verse of Bhagavad Gita.