Was the rapture mentioned anywhere before Paul revealed this mystery in 1 Corinthians 15:51?

No. There is not even a foggiest hint anywhere in the Old Testament, the Gospels, or Acts of a group of people being alive and suddenly taken off the scene, which is called the rapture. This is a Church Age phenomenon revealed through Paul’s writings.

Les explains that 1 Corinthians 15 deals with resurrection, and Paul uses various allusions and allegories from nature to illustrate resurrection from one state to another. Coming to verse 51, Paul says, “Behold I show you a mystery,” meaning it had been a secret. Les emphasizes that there is not a hint, not even the foggiest hint, anywhere up through the Old Testament, all through the Gospels, and into the Book of Acts, of a group of people who are alive and will be suddenly taken off the scene—the rapture. The reason is that it is a Church Age phenomenon, and only Paul writes concerning the Church, so this Church Age doctrine comes from his writing.

1 Corinthians 15:51

Behold, I shew you a mystery; We shall not all sleep, but we shall all be changed,

Was the mystery of the Church Age known before Paul?

No. The mystery of the Church Age was not known before Paul. It was kept secret in the mind of God until He revealed it to the Apostle Paul. Scripture says this revelation is strictly Pauline, found only in Paul’s letters, not in Israel’s promises.