17 Dionysius did not separate the Persons of the Holy Trinity.
'Each of the names I have mentioned is inseparable and indivisible from that next to it. I spoke of the Father, and before referring to the Son I designated Him too in the Father. I referred to the Son—and even if I did not also expressly mention the Father, certainly He was to be understood beforehand in the Son. I added the Holy Spirit, but at the same time I further added both whence and through whom He proceeded. But they are ignorant that neither is the Father, qua Father, separated from the Son—for the name carries that relationship with it—nor is the Son expatriated from the Father. For the title Father denotes the common bond. But in their hands is the Spirit, who cannot be parted either from Him that sent or from Him that conveyed Him: How then can I, who use these names, imagine that they are sundered and utterly separated from one another?' And after a little he goes on, 'Thus then we extend the Monad indivisibly into the Triad, and conversely gather together the Triad without diminution into the Monad.'
Source: De Sententia Dionysii (New Advent)