One of my favorite stories in the Bible is the tale of the woman caught in adultery in John 8. The story doesn't appear in the majority of the manuscripts, but it so fits with the character of Jesus it's never been seriously recommended for removal. We can all quote from the story: "Go and sin no more"; "Let he who is without sin cast the first stone." It's a beautiful teaching on sin, judgment, and forgiveness.
But it's also the source of one of the greater mysteries to go unsolved in Scripture. Twice, in verses 6 & 8, Jesus stoops down and writes in the dirt with his finger. No one knows what he wrote. Theories abound, of course: scripture, the sins of the woman's accusers, the names of their various mistresses, "<her name> is loved," you name it. At the end of the day, though, no one knows for sure. And we'll never know until we can ask Jesus in person.
That's something I plan to do, honestly. There are so many things about our faith we don't know and can't explain. While I would love to have all the answers, I think it's good we don't. God left room for mystery, and the mystery reminds us we are finite and He is infinite, we are limited and He is limitless. His ways are not our ways (Isaiah 55:8-9) -- or, as C.S. Lewis might say, He is not a tame lion, but He is good.
Through the centuries, the Church has declared one such thing to be the mystery of the faith: "Christ has died; Christ is risen; Christ will come again." All three are true, but we may never fully understand any of them. The power of the cross, the empty tomb, and the Second Coming are partly knowable, but their deep truths are as mysterious as the writing in the sand.