Sentence:  [...] and I might not look at the brightness of His face save through a sheen mirror between me and Him that shields my eyes. So I have often seen the Holy <WSD>Threeness</WSD>, Father and Son and Holy Ghost, three in one, and but a little while might I bear the light. 
Meaning of the word marked with <WSD> tags:  In Christian belief, the three persons (personae) of the Godhead: Father, Son and Holy Spirit; Trinity.