34 The words which follow those last cited are, If you know Me, you know My Father also. It is the Man, Jesus Christ, Whom they behold. How can a knowledge of Him be a knowledge of the Father? For the Apostles see Him wearing the aspect of that human nature which belongs to Him; but God is not encumbered with body and flesh, and is incognisable by those who dwell in our weak and fleshly body. The answer is given by the Lord, Who asserts that under the flesh, which, in a mystery, He had taken, His Father's nature dwells within Him. He sets the facts in their due order thus— If you know Me, you know My Father also; and from henceforth you shall know Him, and have seen Him. He makes a distinction between the time of sight, and the time of knowledge. He says that from henceforth they shall know Him Whom they had already seen; and so shall possess, from the time of this revelation onward, the knowledge of that nature, on which, in Him, they long had gazed.
Source: On the Trinity (New Advent)