3 What then? After all these efforts were they tired? Did they leave off? Not at all. They are charging me with innovation, and base their charge on my confession of three hypostases, and blame me for asserting one Goodness, one Power, one Godhead. In this they are not wide of the truth, for I do so assert. Their complaint is that their custom does not accept this, and that Scripture does not agree. What is my reply? I do not consider it fair that the custom which obtains among them should be regarded as a law and rule of orthodoxy.
If custom is to be taken in proof of what is right, then it is certainly competent for me to put forward on my side the custom which obtains here. If they reject this, we are clearly not bound to follow them. Therefore let God-inspired Scripture decide between us; and on whichever side be found doctrines in harmony with the word of God, in favour of that side will be cast the vote of truth. What then is the charge? Two points are advanced at one and the same time in the accusations levelled against me.
I am accused on the one hand of parting the hypostases asunder; on the other of never using in the plural any one of the nouns relating to the Divinity, but of always speaking in the singular number of one Goodness, as I have already said; of one Power; one Godhead; and so on. As to the parting of the hypostases, there ought to be no objection nor opposition on the part of those who assert in the case of the divine nature a distinction of essences. For it is unreasonable to maintain three essences and to object to three hypostases. Nothing, then, is left but the charge of using words of the divine nature in the singulars.
Source: Letters (New Advent)