We in the modern West think of a mystery as something that can't be understood. That's not how the 1st Near Eastern people understood it. Here's Strong's definition of "mystery"
G3466 μυστήριον musterion (mï-stee'-riy-on) n.
a secret or “mystery.”
{through the idea of silence imposed by initiation into religious rites}
[from a derivative of muo “to shut the mouth”]
Notice the first definition is "a secret." A secret can be perfectly understood once revealed.
Here's Abbott-Smith's Manual Greek Lexicon of the New Testament:
μυστήρισν, -ου, τό
(< μυέω),
[in LXX: Da LXX TH Dan 2:18 ff. (H7328), Tob 12:7; Tob 12:11, Jdt 2:2, Wis 2:22; Wis 6:22; Wis 14:15; Wis 14:23, Sir 3:18; Sir 22:22; Sir 27:16-17; Sir 27:21, 2Ma 13:21 *;]
1. that which is known to the μύστης (initiated), a mystery or secret doctrine, mostly in pl., τὰ μ. (Æsch., Hdt., al.).
2. In later writers (Menand., Incert., 168), that which may not be revealed (not, however, as in the modern sense, intrinsically difficult to understand), a secret or mystery of any kind (To, Jth, 2Mac, ll, c.).
3. In NT, of the counsels of God (cf. Th.: Job 15:8, Psa 25:14 for H5475), once hidden but now revealed in the Gospel or some fact thereof;
Again, we can see that God reveals secrets to believers (particularly #3).
One more from Cyclopedia of Biblical, Theological and Ecclesiastical Literature
Mystery
(μυστήριον), a term employed in the Bible (N.T.), as well as in some of the pagan religions, to denote a revealed secret.
The secret is revealed once again. It be hard to find a definition that indicates the ancient Near East would have understood "musterion" any other way.
The Athenasian Creed is usually accepted as a modern Western style mystery, i.e. something that can never be understood. It states in part;
"So the Father is God; the Son is God; and the Holy Ghost is God. And yet they are not three Gods; but one God."
Well, such a statement defies logic and requires the complete abandonment of the normal meaning of words and grammar. But when that happens the gloves come off, and anybody can say anything whether it makes sense of not. That's how we get a son being his own father. Normally we all understand that makes no sense whatsoever. But if we just call it a "mystery" it suddenly becomes an acceptable doctrine.
I don't mean to besmirch any born again child of God regardless of what the believe about Jesus and God. If they confess Jesus as Lord and believe God raised him from the dead (Rom 10:9-10), they are my brother and I have the deepest love and respect for them. I just think that if they believe the Athenasian Creed they put themselves into a position where the scriptures will be largely a closed book. Imagine reading Moby Dick and mixing up the whale with Captain Ahab. It wouldn't make sense and just saying it's a mystery doesn't help at all in getting the message.