34 Thus the Only-begotten Son, Who is in the bosom of the Father, has told us of God, Whom no man has seen. Either disprove the fact that the Son has thus informed us, or else believe Him Who has been seen, Who appeared to them who knew Him not, and became the God of the Gentiles who called not upon Him and spread out His hands before a gainsaying people. And believe this also concerning Him, that they who serve Him are called by a new name, and that on earth men bless Him and swear by Him as true God.
Prophecy tells, the Gospel confirms, the Apostle explains, the Church confesses, that He Who was seen is true God; but none venture to say that God the Father was seen. And yet the madness of heresy has run to such lengths that, while they profess to recognise this truth, they really deny it. They deny it by means of the newfangled and godless device of evading the truth, while making a studied pretence of adhesion to it. For when they confess one God, alone true and alone righteous, alone wise, alone unchangeable, alone immortal, alone mighty, they attach to Him a Son different in substance, not born from God to be God, but adopted through creation to be a Son, having the name of God not by nature, but as a title received by adoption; and thus they inevitably deprive the Son of all those attributes which they accumulate upon the Father in His lonely majesty.
Source: On the Trinity (New Advent)