Ephesians 3:11

Ephesians 3:8-12 (see also Deuteronomy 29:29)
To me, the very least of all saints, this grace was given, to preach to the Gentiles the unfathomable riches of Christ, and to bring to light what is the administration of the mystery which for ages has been hidden in God who created all things; so that the manifold wisdom of God might now be made known through the church to the rulers and the authorities in the heavenly places. This was in accordance with the eternal purpose which He carried out in Christ Jesus our Lord, in whom we have boldness and confident access through faith in Him.

John Calvin writes: “The calling is therefore a certain and specific calling, which seals and ratifies the eternal election of God so as to make manifest what was before hidden in God.” (Concerning the Eternal Predestination of God, p.70, emphasis mine)

Calvin adds: “First he points out the eternity of election, and then how we should think of it. Christ says that the elect always belonged to God. God therefore distinguishes them from the reprobate, not by faith, nor by any merit, but by pure grace; for while they are far away from him, he regards them in secret as his own.” (John: Calvin, The Crossway Classic Commentaries, p.393, emphasis mine) 

​Question: What was “hidden in God”?

Answer: The eternal elect in the Father? No. Rather, the “riches of Christ” in accordance to the “eternal purpose” which He carried out in Christ.

John Calvin: “As if God had not the right to keep His purposes concealed in His own power until He wishes to communicate them to men! What presumption, what madness it is, not to admit that God is wiser than we! Let us remember, therefore, that our rashness must be suppressed whenever the boundless height of the Divine foreknowledge is set before us. This, too, is the reason why he calls them ‘the unreachable riches of Christ’, meaning that this subject, though it exceeds our grasp, deserves reverence and admiration.” (Calvin’s New Testament Commentaries: Galatians, Ephesians, Philippians and Colossians, p.162) 

The mystery of the ages is not Calvinism; it is “Christ in you” which Paul now presented to the Gentiles:

Romans 16:25-26: Now to Him who is able to establish you according to my gospel and the preaching of Jesus Christ, according to the revelation of the mystery which has been kept secret for long ages past, but now is manifested, and by the Scriptures of the prophets, according to the commandment of the eternal God, has been made known to all the nations, leading to obedience of faith; to the only wise God, through Jesus Christ, be the glory forever. Amen.

Ephesians 1:8-10: In all wisdom and insight He made  known to us the mystery of His will, according to His kind intention which He purposed in Him with a view to an administration suitable  to the fullness of the times, that is, the summing up of all things in Christ, things in  the heavens and things on the earth.

Ephesians 3:3-7: By revelation there was made known to me the mystery, as I wrote before in brief. By referring to this, when you read you can understand my insight into the mystery of Christ, which in other generations was not made known to the sons of men, as it has now been revealed to His holy apostles and prophets in the Spirit; to be specific, that the Gentiles are fellow heirs and fellow members of the body, and fellow partakers of the promise in Christ Jesus through the gospel, of which I was made a minister, according to the gift of God’s grace which was given to me according to the working of His power. 

Colossians 1:26-27: The mystery which has been hidden from the past ages and generations, but has now been manifested to His saints, to whom God willed to make known what is the riches of the glory of this mystery among the Gentiles, which is Christ in you, the hope of glory. 

The mystery of the ages has to do with the summing up of all things in Christ. Conversely, the mystery of the ages has absolutely nothing to do with a Calvinists purported self-discovery of being secretly elect in the Father.