On The Holy Trinity by St. Augustine
The question is explained, which had been deferred in the previous book, viz. that God the Father, who begat the Son, His power and wisdom, is not only the Father of power and wisdom, but also Himself power and wisdom; and similarly the Holy Spirit: yet that there are not three powers or three wisdoms, but one power and one wisdom, as there is one God and one essence. Inquiry is then made, why the Latins say one essence, three persons, in God; but the Greeks, one essence, three substances or hypostases: and both modes of expression are shown to arise from the necessities of speech, that we might have an answer to give when asked, what three, while truly confessing that there are three, viz. the Father, and the Son, and the Holy Spirit.