When Arjuna saw the Universal form he mentions some such facts as to why Krsna is considered the Supreme Personality of Godhead:
All the great authorities, such as Asita, Devala, and Vyasa and all the scriptures have accepted Krsna as the Supreme Personality of Godhead.
Not only that, Krsna Himself asserts and explains this in so many ways in Bhagavad Gita, how He ‘s the source of everything and there is no truth beyond him.
He does not simply make these empty statements, but He actually shows His universal form in the battlefield.
All the great mahajanas beginning from Brahma, Narada, Siva, Sukadeva Gosvami and Bhismadeva, glorify Krsna as the Supreme Absolute Truth and are engaged in His devotional service.
Besides, throughout His lila, all of His uncommon, supramundane activities bear testimony to this fact, such as the lifting of Govardhana Hill at the age of seven, showing the universes within His mouth, expanding Himself into all the cowherd boys and calves, marrying 16,108 princesses, etc.
Thus there are innumerable evidences within the Vedas about Krsna’s identity.
In fact, as stated in Bhagavad-gita, the purpose of the Vedas is itself to know Krsna only.
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