Bg 15.19

yo mam evam asammudho
janati purusottamam
sa sarva-vid bhajati mam
sarva-bhavena bharata

Word for word: 
yah — anyone who; mam — Me; evam — thus; asammudhah — without a doubt; janati — knows; purusa-uttamam — the Supreme Personality of Godhead; sah — he; sarva-vit — the knower of everything; bhajati — renders devotional service; mam — unto Me; sarva-bhavena — in all respects; bharata — O son of Bharata.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Whoever knows Me as the Supreme Personality of Godhead, without doubting, is the knower of everything. He therefore engages himself in full devotional service to Me, O son of Bharata.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
There are many philosophical speculations about the constitutional position of the living entities and the Supreme Absolute Truth. Now in this verse the Supreme Personality of Godhead clearly explains that anyone who knows Lord Krishna to be the Supreme Person is actually the knower of everything. The imperfect knower goes on simply speculating about the Absolute Truth, but the perfect knower, without wasting his valuable time, engages directly in Krishna consciousness, the devotional service of the Supreme Lord. Throughout the whole of Bhagavad-gita, this fact is being stressed at every step. And still there are so many stubborn commentators on Bhagavad-gita who consider the Supreme Absolute Truth and the living entities to be one and the same.

Vedic knowledge is called sruti, learning by aural reception. One should actually receive the Vedic message from authorities like Krishna and His representatives. Here Krishna distinguishes everything very nicely, and one should hear from this source. Simply to hear like the hogs is not sufficient; one must be able to understand from the authorities. It is not that one should simply speculate academically. One should submissively hear from Bhagavad-gita that these living entities are always subordinate to the Supreme Personality of Godhead. Anyone who is able to understand this, according to the Supreme Personality of Godhead, Sri Krishna, knows the purpose of the Vedas; no one else knows the purpose of the Vedas.

The word bhajati is very significant. In many places the word bhajati is expressed in relationship with the service of the Supreme Lord. If a person is engaged in full Krishna consciousness, in the devotional service of the Lord, it is to be understood that he has understood all the Vedic knowledge. In the Vaisnava parampara it is said that if one is engaged in the devotional service of Krishna, then there is no need for any other spiritual process for understanding the Supreme Absolute Truth. He has already come to the point, because he is engaged in the devotional service of the Lord. He has ended all preliminary processes of understanding. But if anyone, after speculating for hundreds of thousands of lives, does not come to the point that Krishna is the Supreme Personality of Godhead and that one has to surrender there, all his speculation for so many years and lives is a useless waste of time.