Michael Wilcock writes this helpful paragraph on the use of the word ‘mystery’ in the New Testament:

“A very cursory study of the New Testament use of the word ‘mystery’ shows  that it does not there carry its usual modern sense of ‘puzzle’. It is indeed something hidden, but not in such a way that you can follow a series of clues and eventually find it out; rather, it is a truth which you either know or do not know, depending on whether or not it has been revealed to you. To the initiate, it will never again be a secret; but so long as he was an outsider, it could never be anything else. The ‘mysteries’ of the New Testament are open secrets to every Christian. The ‘mystery of Christ’ spoken of in Ephesians 3:3-6 is a truth which was hidden from ‘men in other generations’, but ‘has now been revealed to his holy apostles’, and Paul in turn has ‘written briefly’ of it to the Ephesians; it is, in a sentence, that ‘the Gentiles are . . . partakers of the promise in Christ Jesus’ along with God’s ancient people the Jews. To Paul and his readers this is no longer a secret.”