Summary: The message of Christ and His work on our behalf is a mystery of God. In fact, Paul speaks of this as "the mystery of godliness."
“Great indeed, we confess, is the mystery of godliness:
He was manifested in the flesh,
vindicated by the Spirit,
seen by angels,
proclaimed among the nations,
believed on in the world,
taken up in glory.” 
Mysteries are a favourite genre in books, movies and television entertainment in contemporary culture; and the New Testament seems to speak of “mysteries” rather frequently. However, whenever the writers of the New Testament spoke of a mystery, they didn’t mean precisely what we mean today when we speak of a mystery. In contemporary language, a mystery is an enigma—an unexplained or unsolved phenomenon—for which the skein must be unraveled in order to discover the cause of some action we have observed. Often, the term refers to an action that is hurtful or harmful, an action resulting in loss, even loss of life, or an action that threatens health and/or life.
The writers of the New Testament used the term mustérion—mystery twenty-eight times. However, in that Greek tongue, the concept of mustérion was a truth that was previously unknown and now revealed. For instance, Jesus explained to the disciples why He spoke in parables. He said, “To you it has been given to know the secrets of the kingdom of heaven, but to them it has not been given. For to the one who has, more will be given, and he will have an abundance, but from the one who has not, even what he has will be taken away. This is why I speak to them in parables, because seeing they do not see, and hearing they do not hear, nor do they understand” [MATTHEW 13:11-13].  The word translated “secrets” in my translation is the Greek term mustérion; the “mysteries of the Kingdom” were deliberately obscured with parables. Disciples would understand what was meant; outsiders would be clueless.
Paul spoke of the recalcitrance of Israel toward the Good News of the Messiah as a mustérion.  According to the Apostle, the Gospel was a mustérion.  That Gentiles would be included in the salvation of God was a mystery.  The lawlessness of the wicked one is a mystery, and especially that this lawlessness is restrained by God’s Spirit is a mystery.  And the truth that not all believers shall die—many will be raptured, translated into the presence of the Risen Master—is a mystery. 
In our text today, Paul speaks of the “mystery of godliness.” The term is intriguing, perhaps the more so as we read his recitation of what appears to have been a hymn that was known to Timothy. I believe he is quoting a hymn that would have been familiar to many Christians during that time.
Before tackling the text, I want to take a moment to speak about Paul’s citation of what I believe to be a hymn. From earliest days, the people of God have been a singing people. John Wesley instructed the circuit riding preachers to carry the Bible and a hymnal. And to this day, the churches of our Lord sing psalms hymns and spiritual songs. In fact, we have instructions to be a singing people. For instance, Paul writes in the Ephesian Letter, “Be filled with the Spirit, addressing one another in psalms and hymns and spiritual songs, singing and making melody to the Lord with your heart, giving thanks always and for everything to God the Father in the name of our Lord Jesus Christ” [EPHESIANS 5:18b-20]. Was this the only place where such instruction was given, it would be enough to encourage us to sing; however, we are instructed elsewhere, “Let the word of Christ dwell in you richly, teaching and admonishing one another in all wisdom, singing psalms and hymns and spiritual songs, with thankfulness in your hearts to God. And whatever you do, in word or deed, do everything in the name of the Lord Jesus, giving thanks to God the Father through him” [COLOSSIANS 3:16, 17].