29 In matters essential to salvation it is not enough to advance the proofs which faith supplies and finds sufficient. Arguments which we have not tested may delude us into a misapprehension of the meaning of our own words, unless we take the offensive by exposing the hollowness of the enemy's proofs, and so establish our own faith upon the demonstrated absurdity of his. The ninth book, therefore, is employed in refuting the arguments by which the heretics attempt to invalidate the birth of God the Only-begotten;— heretics who ignore the mystery of the revelation hidden from the beginning of the world, and forget that the Gospel faith proclaims the union of God and man.
For their denial that our Lord Jesus Christ is God, like God and equal with God as Son with Father, born of God and by right of His birth subsisting as very Spirit, they are accustomed to appeal to such words of our Lord as, Why do you call Me good? None is good save One, even God. They argue that by His reproof of the man who called Him good, and by His assertion of the goodness of God only, He excludes Himself from the goodness of that God Who alone is good and from that true Divinity which belongs only to One.
With this text their blasphemous reasoning connects another, And this is life eternal that they should know You the only true God, and Him Whom You sent, Jesus Christ. Here, they say, He confesses that the Father is the only true God, and that He Himself is neither true nor God, since this recognition of an only true God is limited to the Possessor of the attributes assigned. And they profess to be quite clear about His meaning in this passage, since He also says, The Son can do nothing of Himself, but what He has seen the Father doing. The fact that He can only copy is said to be evidence of the limitation of His nature.
There can be no comparison between Omnipotence and One whose action is dependent upon the previous activity of Another; reason itself draws an absolute line between power and the want of power. That line is so clear that He Himself has avowed concerning God the Father, The Father is greater than I. So frank a confession silences all demur; it is blasphemy and madness to assign the dignity and nature of God to One who disclaims them. So utterly devoid is He of the qualities of true God that He actually bears witness concerning Himself, But of that day and hour knows no one, neither the angels in heaven nor the Son, but God only. A son who knows not his father's secret must, from his ignorance, be alien from the father who knows; a nature limited in knowledge cannot partake of that majesty and might which alone is exempt from the tyranny of ignorance.
Source: On the Trinity (New Advent)