That given, would you say that the conceptualization outlined in the OP might be reversed? Might the Holy Spirit be equated with Saguna Brahman and the Father be equated with Nirguna Brahman?
It might be more accurate to say that the Father, Son and Holy Spirit
are "God with qualities" (akin to
Saguna Brahman), whereas the Divine Essence that they all share as One Godhead
is "God without qualities" (akin to
Nirguna Brahman).
The "
Who" of God, are the Persons, while the "
What" of God is the Essence.
In terms of the 'What,' - and this applies to each Person who is fully God and thus shares the one divine essence - I can affirm nothing. In His Essence, God is unknowable, ineffable and beyond human cognition, comparison or imagination. As St. John of Damascus (675 - 749) explained: "
It is impossible to portray one who is without body: invisible, uncircumscribed and without form."
In terms of 'Who,' I can describe Him by referring to the revealed qualities of the Three Persons.
In
The Book of the Twelve Beguines, the Catholic mystic Ruysbroeck writes:
"...There is a distinction and differentiation, according to our reason, between God and the Godhead, between action and rest.
The fruitful nature of the Persons [of the Trinity], ever worketh in living differentiation.
But the Simple Being of God, according to the nature [essence] thereof, is an Eternal Rest of God and of all created things..."
- Blessed John of Ruysbroeck (1293 - 1381), The Sparkling Stone