12 So, with regard to the Holy Spirit, how can you say that he is the third person, when the persons you mention are innumerable? Or why is he not Jesus himself? And why does Faustus mislead people, in trying to make out an agreement between himself and true Christians, from whom he differs only too widely, by saying, We worship one God under the threefold appellation of the Almighty God the Father, Christ his Son, and the Holy Spirit? Why is the appellation only threefold, instead of being manifold?
And why is the distinction in appellation only, and not in reality, if there are as many persons as there are names? For it is not as if you gave three names to the same thing, as the same weapon may be called a short sword, a dagger, or a dirk; or as you give the name of moon, and the lesser ship, and the luminary of night, and so on, to the same thing. For you cannot say that the First Man is the same as the Mighty Spirit, or as the World-Holder, or as the giant Atlas. They are all distinct persons, and you do not call any of them Christ.
How can there be one Deity with opposite functions? Or why should not Christ himself be the single person, if in one substance Christ hangs on the trees, and was persecuted by the Jews, and exists in the sun and moon? The fact is, your fancies are all astray, and are no better than the dreams of insanity.
Source: Reply to Faustus the Manichaean (New Advent)