27 The Son speaks thus:— For the works which the Father has given Me to finish, the same works which I do, bear witness of Me that the Father has sent Me; and the Father Himself which has sent Me has borne witness of Me. God the Only-begotten proves His Sonship by an appeal not only to the name, but to the power; the works which He does are evidence that He has been sent by the Father. What, I ask, is the fact which these works prove? That He was sent. That He was sent, is used as a proof of His sonlike obedience and of His Father's authority: for the works which He does could not possibly be done by any other than Him Who is sent by the Father.
Yet the evidence of His works fails to convince the unbelieving that the Father sent Him. For He proceeds, And the Father Himself which has sent Me has borne witness of Me; and you have neither heard His voice nor seen His shape. What was this witness of the Father concerning Him? Turn over the pages of the Gospels and review their contents. Read us other of the attestations given by the Father beside those which we have heard already; This is My beloved Son, in Whom I am well pleased, and You are My Son.
John, who heard these words, needed them not, for He knew the truth already. It was for our instruction that the Father spoke. But this is not all. John in the wilderness was honoured with this revelation; the Apostles were not to be denied the same assurance. It came to them in the very same words, but with an addition which John did not receive. He had been a prophet from the womb, and needed not the commandment, Hear Him. Yes; I will hear Him, and will hear none but Him and His Apostle, who heard for my instruction.
Even though the books contained no further witness, borne by the Father to the Son, than that He is the Son, I have, for confirmation of the truth, the evidence of His Father's works which He does. What is this modern slander that His name is a gift by adoption, His Godhead a lie, His titles a pretence? We have the Father's witness to His Sonship; by works, equal to the Father's, the Son bears witness to His own equality with the Father. Why such blindness to His obvious possession of the true Sonship which He both claims and displays.
It is not through condescending kindness on the part of God the Father that Christ bears the name of Son; not by holiness that He has earned the title, as many have won it by enduring hardness in confession of the faith. Such sonship is not of right; it is by a favour, worthy of Himself, that God bestows the title. But that which is indicated by This, and My, and Hear Him, is different in kind from the other. It is the true and real and genuine Sonship.
Source: On the Trinity (New Advent)