Scripture explains what the Godhead (Trinity) is very clearly. There is no 'Mystery'.
The problem is that almost every person on the planet is unwilling to receive the Revelation.
I know the reason, but that's another topic entirely.
And since most are unwilling to accept the obvious, they will never see it and thus it always remains a Mystery to them.
The Godhead is described quite plainly for all to Witness here...
Romans 1:20
"For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse"
Again, most reading this verse have been conditioned to *not* see the obvious. Satan of course has done this on purpose as to not expose the masses to such profound Truths.
Having said that...
Perhaps there may be One here in which the 'programming' doesn't work. For you, I shall now give further explanation.
The 'Invisible Things of Him' is the part of God we are not allowed to see lest we die. It is the 'Word' that creates our reality. Call it 'Software' if that helps One to understand.
Because of this, God must manifest himself in such a way that we in the physical realm can still understand and interact with him.
This is where the Godhead comes in. We are able to see the 'Invisible Things' via the Father, Word and Holy Ghost.
Moreover... the Father, Word and Holy Ghost are the 'Things that are made'.
Does anyone here understand? I mean REALLY understand the literal meaning of this?
'Things that are made' is everything we see around us. It is the Physical World of Gas, Liquid, Solid, Plasma etc.
In other words, the Things that are made is ordinary Matter.
What is ordinary Matter?
The Atom...
Father = Neutron
Word = Proton
Holy Ghost = Electron
And these three are One... as in One Godhead and generic construct of every Atom in existence.
That is your big 'Mystery' revealed. The Godhead is the Biblical term for the Atom.
And as I said, only a small handful of people on the planet are willing to receive this Revelation even though God spelled it out as precisely and succinctly as possible.
We are made in the image of God. Out of Atoms.
God Bless.