“…and to bring to light what is the administration of the mystery which for ages has been hidden in God who created all things…” (Ephesians 3:9)
The “mystery” was hidden “in GOD”. This mystery could not have been discovered by studying the Old Testament. It was revealed to the Apostle Paul by Christ Jesus after His ascension. He then gave Paul the job of preaching this hidden mystery.
“I, Paul, a prisoner for Christ Jesus on behalf of the nations— assuming that you have heard of the stewardship of God's grace that was given to me for you, how the mystery was made known to me by revelation, as I have written briefly. When you read this, you can perceive my insight into the mystery of Christ, which was not made known to the sons of men in other generations as it has NOW been revealed to his holy apostles and prophets by the Spirit…” (Ephesians 3:1-5)
So, what was the Mystery? It was that in the GRACE of God, all people (Jew and Gentile) will become joint members of the body of Christ (the church), not just Israel.
“This mystery is that the nations are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel.” (Ephesians 3:6)
“To me, though I am the very least of all the saints, this grace was given, to preach to the nations the unsearchable riches of Christ, and to bring to light for EVERYONE what is the plan of the mystery hidden for ages in God who created all things, so that through the church the manifold wisdom of God might now be made known to the rulers and authorities in the heavenly places. This was according to the eternal purpose that He has realized in Christ Jesus our Lord.” (Ephesians 3:7-11)
The revelation of this mystery of the Salvation of every human in every nation by the grace of God would make known to the “rulers and authorities in heavenly places” (angelic beings) the wisdom of God.
"...we have set our hope on the living God, who is the Savior of everyone, and especially of those who believe." (1 Timothy 4:10)
“Behold, I tell you a mystery…the dead will be raised…" (1 Corinthians 15: 51-52)
"...our citizenship is in heaven, and we eagerly await a Savior from there, the Lord Jesus Christ, who, by the power that enables Him to subject all things to Himself, will transform our lowly bodies to be like His glorious body." (Phillipians 3:20-21)
I will continue to vote as a citizen of this country for imperfect leaders. However, my eternal citizenship is in the Kingdom of God, a New Heaven and New Earth of perfect righteousness promised to the body of Christ after this mortal life ends!