So then, when Paul says, I don't want you ignorant, I show you a mystery, this is the revelation.Oh Boy - here we go....
3466 mystḗrion (the root of the English term, "mystery") – mystery. In the Bible, a "mystery" (3466 /mystḗrion) is not something unknowable. Rather, it is what can only be known through revelation, i.e. because God reveals it.
More OT musterion
I will declare Your name to My brethren;
In the midst of the assembly I will praise You.
You who fear the Lord, praise Him!
All you descendants of Jacob, glorify Him,
And fear Him, all you offspring of Israel!
Psalm 22
Much love!