In Genesis 2:24 we are told that a man shall leave his father and mother and cleave to his wife, and that the two shall become one flesh. The reason for it is the first poem of Scripture, the first words of man: "bone of my bones, and flesh of my flesh; she shall be called אסה (i-sha) for from 'מעיס לקחה זאת' (me-ish lu-ko-cha zot: man was taken she)."
The curious part comes in the NT reference of Paul's, in Ephesians 5:31 (by the way: Paul is not the name Saul was given when he stopped using Biblical heroes for target practice a la Braveheart; Paul is the Greek [Gentile] form of the Hebrew Saul). Because Paul, in his wonderful Greek (for this reason a man will leave his father and mother and "προσκολληθησεται προσ την γυναικα:" shall be cleaved to his wife), adds on a lovely little addendum: Ephesians 5:32. "This is a great mystery, but I speak of Christ and the church."
I don't know how many times I have read that, and it is quite simply put, but I don't think it ever really sank in. Just think of the ramifications of this analogy: a son leaving his family is compared to Christ's incarnation. Please, if one of my four faithful readers (moving up in the world, aren't we?) notices some random bit of heresy thrown in, don't start stacking the faggots: it's actually hard to speak of things like this without coming dangerously close to an edge somewhere or other, and I'd really prefer not to be burned at the stake quite yet.
Now, we always gravitate to the second half of the Genesis reference: the great mystery is Christ and the Church as husband and wife, and this is true, and this is glorious, and this is astounding. But we completely gloss over what had to happen for this even to be possible: Christ, the Second Person of the Godhead, had to leave His Father and--figuratively--Mother (forgive me and keep in mind, I hate The Shack), and become human. On top of this, He never leaves that humanity behind. He is glorified, yes, He is Deity, yes, but he never leaves His human flesh behind.
Allow me to quickly qualify the rest of this post: God is God, and we aren't, and we never will be, and He'll never not be. All clear? There is an infinite distance between God and us, always has been, always will be. God to us is infinitely greater than we are to whatever is eaten by the bacteria dying at the hands of our white blood cells right now, and always will be, okay? I don't even like fire.
So, this is the narrative arc of the story. God created humans in His Image, to become like Him. Not only did we not continue in His Image, but we defiled and defaced it. Now, picture this: you have the Infinite, the Creator, the Αρχε of all: He can do whatever He wants, He can write us off and start over from scratch; He can skip the whole process and simply create the result; He can do things that we could never even think of, or He could just say forget it. After all, it's not as if He needs us for anything. But no. He does none of this. He completes His purpose; He makes us like Him. How? By making Himself like us.
As humans, we were created to be like God--it was no false temptation--but we fell where He stood: "being equal with God, He did not consider equality with God something to be grasped..." Humans were created to ascend into a (lesser, finite, limited) type of equality with God, but we fell. So God jumps down into a (greater, dual natured μυστεριον) type of equality with us so that He might become a fitting Bridegroom, then ascends into what we were supposed to be, that He might lead us thence, and the story be completed in a Wedding.
This is indeed a Great Mystery; this is indeed The Great Mystery. Let us be grateful: our story does not start with birth and end with death. Our story starts with birth, dies, is reborn into a greater life, and then gets eternally married.