As mentioned above, the Svetasvatara Upanisad (3.19) states:

This Vedic mantra clearly states, purusam mahantam. The word purusa means "person." That person is confirmed in the Bhagavad-gita by Arjuna when he addresses Krsna, purusam sasvatam: "You are the original person." (Bg. 10.12) This purusam mahantam is Sri Krsna. His hands and legs are not mundane, but are completely transcendental. However, when He comes, fools take Him to be an ordinary person (avajananti mam mudha manusim tanum asritam). One who has no Vedic knowledge, who has not studied the Vedas from a bona fide spiritual master, does not know Krsna. Therefore he is a mudha. Such fools take Krsna to be an ordinary person (param bhavam ajanantah). They do not actually know what Krsna is. Manusyanam sahasresu kascid yatati siddhaye. It is not possible to understand Krsna simply by studying the Vedas perfectly. One must have the mercy of a devotee (yat-padam). Unless one is favored by a devotee, he cannot understand the Supreme Personality of Godhead. Arjuna confirms this in the Bhagavad-gita (10.14): "My Lord, it is very difficult to understand Your personality." The less intelligent class of men cannot understand the Supreme Personality of Godhead without being favored by His devotee. Therefore the Bhagavad-gita (4.34) contains another injunction:

One has to approach a bona fide spiritual master and surrender to him. Only then can one understand the Supreme Personality of Godhead as a person.