to be let in on a secret; to come to know firsthand, through hard experience, what only an insider knows how to do or endure
The word comes from an older verb meaning "to close" the eyes or the lips, the hush that falls on someone who has been let in on something not meant to be spoken about. In classical Greek it was the technical term for being brought into the secret religious rites of the day, the so-called mystery cults, where an initiate saw and heard things outsiders never could. Over the centuries the word softened, and by New Testament times it was also used simply for being schooled in a skill or a way of life by going through it, learning a trade's secrets or life's hard lessons the way only experience can teach.
Paul uses it exactly once, in Philippians 4:12: "I have learned the secret" of being content whether well fed or hungry, in plenty or in need. He stacks it beside the plain verb for learning (μανθάνω), and it belongs to the same word family as the noun for a revealed secret (μυστήριον). The point is not book learning but knowledge won by living through the ups and downs he names.
The twist is that the "secret" Paul has been let in on is the opposite of what the old mysteries promised. Theirs was hidden knowledge reserved for a chosen few who paid to be admitted. His is contentment through Christ who strengthens him, and it is open to anyone.
Disputed
How much of the word's background in pagan mystery-initiation Paul still intends in its one New Testament use (Philippians 4:12): a live, deliberate echo, or a faded everyday idiom for hard-won learning.
- majorityA faded, everyday idiom: Paul simply means he has learned the secret of contentment by experience, with no working reference to the mystery cults.; LSJ, s.v. μυέω II; Gordon D. Fee, Paul's Letter to the Philippians (NICNT; Eerdmans, 1995), 431-432; Peter T. O'Brien, The Epistle to the Philippians (NIGTC; Eerdmans, 1991), 521, 525
- minorityA live metaphor deliberately reached for: writing to a colony steeped in pagan cult, Paul borrows the technical verb of initiation and reinscribes it under the sufficiency of Christ.; J. B. Lightfoot, St Paul's Epistle to the Philippians (Macmillan, 1868); Paul A. Holloway, Philippians (Hermeneia; Fortress, 2017)