Mystery is "Christ in you" the hope of glory.
I do not understand why you quote the verse in Ephesians 3
but then do not follow what Paul says is the mystery that he is speaking about.
"That the gentiles should be fellow heirs, and of the SAME BODY and partakes of His promise in Christ by the Gospel" then expanded on after .
Thats verse 6. the Emphasis (SAME BODY) is mine.
There is as, 'Complete' has listed many mysteries . But the mystery of Ephesians 3 is as Paul says of it .
and we should not mix all the mysteries up as all being the same mystery .
For it will only create confusion later in our thinking.
Even as God in the beginning divided everything or separated .
So also will he at the end gather all things that are of God together In Christ and be subject to Him .
For by Him were all things made and there is not anything that was made that was not made or created by Him .
and when all things have been made subject to the Son then the Son will subject Himself to the Father that God may be all in all.
in Christ
gerald