The death, burial and resurrection of the Messiah, Jesus Christ, was the mystery. Even His disciples had no clue because it was hid from them on purpose as it was everyone else since the world began.
Luke 18
31 Then he took unto him the twelve, and said unto them, Behold, we go up to Jerusalem, and all things that are written by the prophets concerning the Son of man shall be accomplished.
32 For he shall be delivered unto the Gentiles, and shall be mocked, and spitefully entreated, and spitted on:
33 And they shall scourge him, and put him to death: and the third day he shall rise again.
34 And they understood none of these things: and this saying was hid from them, neither knew they the things which were spoken.