Christians often misuse the word “mystery” to describe something that they can’t understand or more particularly something that the believe God has not revealed.
The Greek word, “musterion”, translated in English as “mystery” meant something totally different. Rather than being something hidden to believers, a “mystery” is something which is only hidden now to unbelievers. A “mystery” in the Bible is something now revealed to the believer.
Follow the texts:
Mar 4:11 And He said to them, To you it has been given to know the mystery of the kingdom of God. But to these, those outside, all things are being given in parables,
Rom 11:25 For I do not want you to be ignorant of this mystery, brothers, so that you may not be wise within yourselves, that hardness in part has happened to Israel until the fullness of the nations comes in;
Rom 16:25 Now to Him who is able to establish you according to my gospel, and the proclaiming of Jesus Christ, according to the revelation of the mystery having been kept unvoiced during eternal times, but now has been made plain, and by prophetic Scriptures, according to the commandment of the everlasting God, made known for obedience of faith to all the nations;
Check out the rest with a concordance. There are 22 NT uses of “mystery”. They all refer to something that is revealed, not something unknown or hidden. Quite the contrary.