3 Hear, therefore, the word of the Lord, brethren; see how He confirmed to them both what they said, “We know this man whence he is,” and also what they said, “When Christ comes, no man knows whence He is. Then cried Christ in the temple, saying, You both know me, and you know whence I am: and I am not come of myself, but He that sent me is true, whom you know not.” That is to say, you both know me, and you know me not; ye both know whence I am, and you know not whence I am.
You know whence I am: Jesus of Nazareth, whose parents also ye knew. For in this case, the birth of the Virgin alone was hidden, to whom, however, her husband was witness; for the same was able faithfully to declare this, who was also able as a husband to be jealous. Therefore, this birth of the Virgin excepted, they knew all that in Jesus pertains to man: His face was known, His country was known, His family was known; where He was born was to be known by inquiry. Rightly then did He say, “You both know me, and you know whence I am,” according to the flesh and form of man which He bore; but according to His divinity, “And I am not come of myself, but He that sent me is true, whom you know not;” but yet that you may know Him, believe in Him whom He has sent, and you will know Him.
For, “No man has seen God at any time, except the only-begotten Son, who is in the bosom of the Father, He has declared Him:” and, “None knows the Father but the Son, and he to whom the Son wills to reveal Him.”
Source: Tractates on the Gospel of John (New Advent)