However a great man one may be, he is sure to commit mistake. I shall give you one tangible example in the life of Mahatma Gandhi. You know he was a very great man, political leader in India. So he was also very God-fearing man, a very nice soul. But he also committed mistakes so many times.
So to err is human. This is a fact in every person. Therefore, because we commit mistake, because we are sometimes illusioned, and because we have got a propensity of cheating others, and because our senses are imperfect, therefore, simply by mental speculation it is not possible to realize God.
Then how one can realize God? That is mentioned in the Bhagavad-gītā and other literature, that by the mercy of God one can realize God. So by His causeless mercy, He comes down. Yadā yadā hi dharmasya glānir bhavati bhārata (BG 4.7), it is stated in the Bhagavad-gītā.
So Lord Kṛṣṇa is accepted as the Supreme Personality of Godhead at least by the great ācāryas of India. Even, as I was speaking of Śaṅkarācārya, he was impersonalist, but still he has admitted in his commentary on Śrīmad-Bhāgavatam that sa kṛṣṇa bhagavān svayam. He has accepted it.
In the beginning of his commentary he says, nārāyaṇaḥ para avyaktāt: "Nārāyaṇa, the Supreme Personality of Godhead, is beyond this material creation." And in next page he has admitted, "That Supreme Personality of Godhead Nārāyaṇa is Kṛṣṇa, who is born as the son of Devakī and Vasudeva."