'Mystery' in the Bible

And when I came to you, brothers and sisters, proclaiming to you the testimony of God [concerning salvation through Christ], I did not come with superiority of speech or of wisdom [no lofty words of eloquence or of philosophy as a Greek orator might do];

Listen very carefully, I tell you a mystery [a secret truth decreed by God and previously hidden, but now revealed]; we will not all sleep [in death], but we will all be [completely] changed [wondrously transformed],