What is the mystery that Paul says he speaks in 1 Corinthians 2:7?

Les explains that Paul writes to the church at Corinth, predominantly Gentile believers, and Paul always writes to believers, though his message would also go to unbelievers. In 1 Corinthians 2:7 Paul says, “but we speak the wisdom of God in a mystery, a secret, even the hidden wisdom, which God ordained before the world unto our glory.” Les notes that Paul uses the plural pronoun “our” instead of “my” because God knew this apostle would become a writer of Scripture. Verse 8 adds that none of the princes of this world knew it—the Jewish leaders did not know who He was, and Rome certainly did not know—for had they known, they would not have crucified the Lord of glory.