THE MYSTERY
A mystery, by definition (and certainly in Biblical usage) is something which is not easily understood, but that will eventually be revealed to those who search and wait.
Think about it. How interesting do you think it would be if Sir Arthur Conan Doyle wrote his Sherlock Holmes stories such that Holmes never explains what had been a mystery? What if the books simply ended with Holmes declaring, "I know what happened." The End?
Similarly, God has wrapped the cure to spiritual cancer within a mystery in order that those who shared His passion would continue to listen until it is revealed.