The word "mystery" is used only 25 times in Scripture: six times in the Old Testament (book of Daniel only, involving the dreams of pagan kings) and 19 times in the New Testament (with only four of those uses in the book of Revelation - a mystery to everybody!). There are two things that strike me about these facts. First, God was actually a greater mystery to people under the old covenant than the new, although he is never described that way. Those followers never saw God incarnate (made human). The closest they got was a cloud by day, a pillar by night and an Ark in the holy of holies (oh, and Moses getting mooned! Ex. 33:23). Second, Paul and John, men who actually saw Jesus in person, were the only New Testament contributors to call Christ and his gospel a "mystery."
I find it amusing that Paul wrote three letters within one to two year's time (perhaps even from the same jail cell) with three very different perceptions on the mystery of Christ:
In Ephesians (likely written first), he writes, "Surely you have heard about...the mystery made known to me by revelation.... In reading this [letter], then, you will be able to understand my insight into the mystery of Christ....This mystery is that through the gospel, [we] share in the promise of Christ Jesus" (3:2-6).
In Colossians (possibly written second), he writes, "My purpose is that they may...have the full riches of complete understanding, in order that they may know the mystery of God, namely, Christ" (2:2).It seems as though Paul becomes less certain of what he knows about the mystery of Christ, and more certain that he has more to learn, more to absorb, more to experience. A process most of us call maturity.
Yet, in Philippians (probably the last of these prison letters), he writes, "I want to know Christ and the power of his resurrection and the fellowship of sharing in his sufferings" (3:10).
Now, I'm not suggesting that Paul was making up something or speaking arrogantly of what he didn't know in his earlier letters. I believe with assurance that God had revealed a great deal to him about His nature, character and will. But I also believe that as God revealed himself, Paul realized (as most of us do) the true "otherness" and incomprehensibility of Holy God. A year or so later, in 1 Timothy, Paul even wrote that "without question" the mystery of Christ is "great" (3:16).
Therefore, rather than trying to "figure out" this mystery, wouldn't we be better served to allow God to reveal himself to us in his way, and in his time? Yes, God gave us a mind to use for comprehension and deduction; thinking about Who God is is an incredible use of those properties and energies. But, unless those efforts are rooted in faith, love and reverence, God is merely a study for science or philosophy, something to be proven or analyzed.
What God is after is a relationship with each of us, through which he reveals himself. Still better, he has given us the Holy Spirit to "[make] known to us the mystery of his will, [that is] to bring all things in heaven and on earth together under Christ" (Ephesians 1:9-10).
In other words, it's Ok for God to be a mystery. Couples married for 50+ years still sometimes find surprises in one another. Healthy relationships aren't dependent on knowing all about the other, they're dependent on wanting to know about them as you live life together.
What's God going to show you about himself today that will make him less of a mystery than he was yesterday? Let's give him the freedom in our relationship with Him to be Who He truly is.