Augustine of Hippo
Catena Aurea · Johannesevangelium 17
These words spake Jesus, and lifted up his eyes to heaven, and said, Father, the hour is come; glorify thy Son, that thy Son also may glorify thee: As thou hast given him power over all flesh, that he should give eternal life to as many as thou hast given him. And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent. I have glorified thee on the earth: I have finished the work which thou gavest me to do. And now, O Father, glorify thou me with thine own self with the glory which I had with thee before the world was.
(vi. de Tr. c. 9) Dismissing then the Arians, let us see if we are forced to confess, that by the words, That they may know Thee to be the only true God, He means us to understand that the Father only is the true God, in such sense as that only the Three together, Father, Son, and Holy Ghost, are to be called God? Does our Lord’s testimony authorize us to say that the Father is the only true God, the Son the only true God, and the Holy Ghost the only true God, and at the same time, that the Father, Son, and Holy Ghost together, i. e. the Trinity, are not three Gods, but onea true God?