Ephesians 3:2-10 (ESV)
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. This mystery is that the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel. Of this gospel I was made a minister according to the gift of God's grace, which was given me by the working of his power. To me, though I am the very least of all the saints, this grace was given, to preach to the Gentiles 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.
W.A. Criswell Believer's Study Bible
Paul's meaning of a mystery (musterion) is the exact opposite of its use in the mystery religions, in which esoteric teachings were communicated to an elite few, who were enjoined never to divulge the secrets. Paul uses the term to herald the publication of the long-hidden secrets of God to the entire world. Chief among these is the fact that God is now welcoming Gentiles into His kingdom and affirming them as having equal standing before Him with Jewish believers in Christ.
William Gurnall - The Mystery of the Gospel in The Christian in Complete Armour
http://www.ccel.org/g/gurnall/armour/gurnal08b.htm
Ephesians 6:19b That I may open my mouth boldly, to make known the mystery of the gospel.
The (hardening) of the Jews and calling of the Gentiles is called a ‘mystery,’ (Romans 11:25); the wonderful change of those that shall be upon the earth at the end of the world, (1 Cor. 15:51); the incarnation, resurrection, and ascension of Christ, (1 Tim. 3:16). Sometimes it is used for the whole body of the gospel; as to the doctrine of it, called a ‘mystery of faith,’ (1 Tim. 3:9); as to the purity of its precepts and rules for a holy life, a ‘mystery of godliness;’ as to the author, subject, and end of it, called ‘the mystery of Christ,’ (Ephesians 3:4)—it was revealed by him, treats of him, and leads souls to him; and lastly, in regard of the blessed reward it promiseth to all that sincerely embrace it, called ‘the mystery of the kingdom of God,’ (Mark 4:11).