Bg 18.75

vyasa-prasadac chrutavan
etad guhyam aham param
yogam yogesvarat Krishnat
saksat kathayatah svayam

Word for word: 
vyasa-prasadat — by the mercy of Vyasadeva; srutavan — have heard; etat — this; guhyam — confidential; aham — I; param — the supreme; yogam — mysticism; yoga-isvarat — from the master of all mysticism; Krishnat — from Krishna; saksat — directly; kathayatah — speaking; svayam — personally.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
By the mercy of Vyasa, I have heard these most confidential talks directly from the master of all mysticism, Krishna, who was speaking personally to Arjuna.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Vyasa was the spiritual master of Sanjaya, and Sanjaya admits that it was by Vyasa’s mercy that he could understand the Supreme Personality of Godhead. This means that one has to understand Krishna not directly but through the medium of the spiritual master. The spiritual master is the transparent medium, although it is true that the experience is still direct. This is the mystery of the disciplic succession. When the spiritual master is bona fide, then one can hear Bhagavad-gita directly, as Arjuna heard it. There are many mystics and yogis all over the world, but Krishna is the master of all yoga systems. Krishna’s instruction is explicitly stated in Bhagavad-gita – surrender unto Krishna. One who does so is the topmost yogi. This is confirmed in the last verse of the Sixth Chapter. Yoginam api sarvesam.

Narada is the direct disciple of Krishna and the spiritual master of Vyasa. Therefore Vyasa is as bona fide as Arjuna because he comes in the disciplic succession, and Sanjaya is the direct disciple of Vyasa. Therefore by the grace of Vyasa, Sanjaya’s senses were purified, and he could see and hear Krishna directly. One who directly hears Krishna can understand this confidential knowledge. If one does not come to the disciplic succession, he cannot hear Krishna; therefore his knowledge is always imperfect, at least as far as understanding Bhagavad-gita is concerned.

In Bhagavad-gita, all the yoga systems – karma-yoga, jnana-yoga and bhakti-yoga – are explained. Krishna is the master of all such mysticism. It is to be understood, however, that as Arjuna was fortunate enough to understand Krishna directly, so, by the grace of Vyasa, Sanjaya was also able to hear Krishna directly. Actually there is no difference between hearing directly from Krishna and hearing directly from Krishna via a bona fide spiritual master like Vyasa. The spiritual master is the representative of Vyasadeva also. Therefore, according to the Vedic system, on the birthday of the spiritual master the disciples conduct the ceremony called Vyasa-puja.