Sentence:  As Gregory of Nyssa had explained, the three <WSD>"hypostases</WSD>" of Father, Son, and Spirit were not objective facts but simply “terms that we use” to express the way in which the “unnameable and unspeakable” divine nature ("ousia") adapts itself to the limitations of our human minds. 
Meaning of the word marked with <WSD> tags:  The essential person, specifically the single person of Christ (as distinguished from his two ‘natures', human and divine), or of the three ‘persons' of the Trinity (sharing a single ‘essence'). (from 16th c.)