What is the mystery of faith?
The phrase “mystery of faith” appears only once in the scripture. That is in 1 Timothy 3:9, describing the qualities of deacons. Later in that same chapter (verse 16) Paul explains what that “mystery” is. “And without controversy great is the mystery of godliness: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory.”
The Mystery Religions were common at that time, and their predominant feature was that the secrets of the religion could only be fully known by those who were specially enlightened. We see the same idea in many cults today. Paul wrote the letters to the Ephesians and Colossians to tell those who were being swayed by these religions that the “mystery” of faith or of God was something that could be known to everyone, not just the religious elite. John was combating the same idea in the Gnostics or Proto-Gnostics in his letters (1 John, 2 John, 3 John), as seen by his emphasis on the word “know.”
Paul told the Ephesians that the mystery (or part of it) was that the gospel was to be preached to the gentiles (Ephesians 3). He told the Colossians that the mystery was to the gentiles as well as the Jews, and that it was “Christ in you, the hope of glory.” (Colossians 1:27)