4 I have quite a little difficulty in meeting the second charge. Whoever condemns those who assert that the Godhead is one, must of necessity agree with all who maintain many godheads, or with those who maintain that there is none. No third position is conceivable. The teaching of inspired Scripture does not allow of our speaking of many godheads, but, wherever it mentions the Godhead, speaks of it in the singular number; as, for instance, “in him dwells all the fullness of the Godhead bodily.” And again; “for the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead.” If, then, to multiply godheads is the special mark of the victims of polytheistic error, and to deny the Godhead altogether is to fall into atheism, what sense is there in this charge against me of confessing one Godhead?
But they make a plainer disclosure of the end they have in view; namely, in the case of the Father to agree that He is God, and consenting in like manner that the Son be honoured with the attribute of Godhead; but to refuse to comprehend the Spirit, though reckoned with Father and with Son in the idea of Godhead. They allow that the power of the Godhead extends from the Father to the Son, but they divide the nature of the Spirit from the divine glory. Against this view, to the best of my ability, I must enter a brief defence of my own position.
Source: Letters (New Advent)