26 And thus, although nothing but a sincere belief that these names are truly significant,— that, when we read, This is My Son and My Father, the words really indicate Persons of Whom, and to Whom, they were spoken— can make them intelligible, yet, lest it be supposed that Son and Father are titles the one merely of adoption, the other merely of dignity, let us see what are the attributes attached, by the Son Himself, to His name of Son.
He says, All things are delivered Me of My Father, and no one knows the Son but the Father, neither knows any the Father save the Son, and he to Whom the Son will reveal Him. Are the words of which we are speaking, This is My Son and My Father, consistent, or are they not, with No one knows the Son but the Father, neither knows any the Father save the Son? For it is only by witness mutually borne that the Son can be known through the Father, and the Father through the Son.
We hear the voice from heaven; we hear also the words of the Son. We have as little excuse for not knowing the Son, as we have for not knowing the Father. All things are delivered unto Him; from this All there is no exception. If They possess an equal might; if They share an equal mutual knowledge, hidden from us; if these names of Father and Son express the relation between Them, then, I demand, are They not in truth what They are in name, wielders of the same omnipotence, shrouded in the same impenetrable mystery? God does not speak in order to deceive. The Fatherhood of the Father, the Sonship of the Son, are literal truths. And now learn how facts bear out the verities which these names reveal.
Source: On the Trinity (New Advent)