21 They are not, therefore, acquitted of blame for not recognising the testimony; for the works of Christ are the Father's testimony concerning Him. Nor can they plead ignorance of the testimony on the ground that they had not heard the voice of the Testifier, nor seen His form, nor had His word abiding in them. For immediately after the words, You have neither heard His voice at any time, nor seen His form, and you have not His word abiding in you, He points out why the voice was not heard, nor the form seen, and the word did not abide in them, though the Father had testified concerning Him: For Whom He sent, Him ye believe not; that is, if they had believed Him, they would have heard the voice of God, and seen the form of God, and His word would have been in them, since through the unity of Their nature the Father is heard and manifested and possessed in the Son.
Is He not also the expression of the Father, since He was sent from Him? Does He distinguish Himself by any difference of nature from the Father, when He says that the Father, testifying of Him, was neither heard, nor seen, nor understood, because they did not believe in Him, Whom the Father sent? The Only-begotten God does not, therefore, separate Himself from God when He confesses God the Father; but, proclaiming by the word “Father” His relationship to God, He includes Himself in the honour due to God.
Source: On the Trinity (New Advent)