"Yes, it is a fact that no one can see Krsna without being a pure devotee of the Lord. Every living entity is the son of god, but not all of the sons are faithful to their Father. This is explained in Bhagavad Gita. So one who turns away from his father naturally cannot see his father; but the faithful son can see his father at any time. Under the circumstances, we can understand the word of John because amongst all the people of that place Jesus Christ was the only faithful son of God, Krsna. Lord Jesus says (John 6:46), “Not that any man hath seen the father, save he which is of God, he hath seen the Father.” So one who is a devotee of the Lord in pure love, who teaches others how to love God is the messenger of God and he can see God without any doubt."
Srila Prabhupada's Letter to: Gerald -- ISKCON: Akash Ganga Bldg; 89 Bhulabhai Desai Road; Bombay-26 India 19th February, 1971