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; that is, how the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the Gospel.
The Church's Eucharistic celebration is called the "Sacred Mysteries" because in the Body of Christ the mystery of Christ is made manifest and all people, Gentile and Jew alike, are made "fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the Gospel." What's so mysterious about that? Nothing — to us, because we tend to take revelation for granted and forget that it is revelation. But for the Apostles who knew the score, this mystery was known for what it was: a truth hidden in the deepest counsels of God for ages until Christ made it known. Because Christ made it known, people from anywhere in the world can have access to the very same God who once revealed Himself only to Israel and can participate in His divine nature in a way that the Old Testament prophets and seers could only dream of. Today, go to Mass and participate in the mystery of His divine nature.