'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];

But we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory:

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