Here are a few words from the New Testament to give larger context to the premise of "mystery."
Matthew 13:10-12
And the disciples came and said to Him, "Why do You speak to them in parables?" Jesus answered them, "To you it has been granted to know the mysteries of the kingdom of heaven, but to them it has not been granted. "For whoever has, to him more shall be given, and he will have an abundance; but whoever does not have, even what he has shall be taken away from him.
Romans 11:25-26
For I do not want you, brethren, to be uninformed of this mystery—so that you will not be wise in your own estimation—that a partial hardening has happened to Israel until the fullness of the Gentiles has come in; and so all Israel will be saved...
Romans 16:25-26
Now to Him who is able to establish you according to my gospel and the preaching of Jesus Christ, according to the revelation of the mystery which has been kept secret for long ages past, but now is manifested, and by the Scriptures of the prophets, according to the commandment of the eternal God, has been made known to all the nations, leading to obedience of faith;
1 Corinthians 2:6-8
Yet we do speak wisdom among those who are mature; a wisdom, however, not of this age nor of the rulers of this age, who are passing away; but we speak God's wisdom in a mystery, the hidden wisdom which God predestined before the ages to our glory; the wisdom which none of the rulers of this age has understood; for if they had understood it they would not have crucified the Lord of glory;
1 Corinthians 4:1
Let a man regard us in this manner, as servants of Christ and stewards of the mysteries of God.
1 Corinthians 15:50-52
Now I say this, brethren, that flesh and blood cannot inherit the kingdom of God; nor does the perishable inherit the imperishable. Behold, I tell you a mystery; we will not all sleep, but we will all be changed, in a moment, in the twinkling of an eye, at the last trumpet; for the trumpet will sound, and the dead will be raised imperishable, and we will be changed.
Ephesians 1:9-10
He made known to us the mystery of His will, according to His kind intention which He purposed in him with a view to an administration suitable to the fullness of the times, that is, the summing up of all things in Christ, things in the heavens and things on the earth.
Ephesians 3:4-10
By referring to this, when you read you can understand my insight into the mystery of Christ, which in other generations was not made known to the sons of men, as it has now been revealed to His holy apostles and prophets in the Spirit; to be specific, that the Gentiles are fellow heirs and fellow members of the body, and fellow partakers of the promise in Christ Jesus through the gospel, of which I was made a minister, according to the gift of God's grace which was given to me according to the working of His power. To me, the very least of all saints, this grace was given, to preach to the Gentiles the unfathomable riches of Christ, and to bring to light what is the administration of the mystery which for ages has been hidden in God who created all things; so that the manifold wisdom of God might now be made known through the church to the rulers and the authorities in the heavenly places.
Ephesians 6:18-20
With all prayer and petition pray at all times in the Spirit, and with this in view, be on the alert with all perseverance and petition for all the saints, and pray on my behalf, that utterance may be given to me in the opening of my mouth, to make known with boldness the mystery of the gospel, for which I am an ambassador in chains; that in proclaiming it I may speak boldly, as I ought to speak.
Colossians 1:25-28
Of this church I was made a minister according to the stewardship from God bestowed on me for your benefit, so that I might fully carry out the preaching of the word of God, that is, the mystery which has been hidden from the past ages and generations, but has now been manifested to His saints, to whom God willed to make known what is the riches of the glory of this mystery among the Gentiles, which is Christ in you, the hope of glory. We proclaim Him, admonishing every man and teaching every man with all wisdom, so that we may present every man complete in Christ.
Colossians 2:1-3
For I want you to know how great a struggle I have on your behalf and for those who are at Laodicea, and for all those who have not personally seen my face, that their hearts may be encouraged, having been knit together in love, and attaining to all the wealth that comes from the full assurance of understanding, resulting in a true knowledge of God's mystery, that is, Christ Himself, in whom are hidden all the treasures of wisdom and knowledge.
Colossians 4:2-4
Devote yourselves to prayer, keeping alert in it with an attitude of thanksgiving; praying at the same time for us as well, that God will open up to us a door for the word, so that we may speak forth the mystery of Christ, for which I have also been imprisoned; that I may make it clear in the way I ought to speak.
2 Thessalonians 2:7
For the mystery of lawlessness is already at work; only he who now restrains will do so until he is taken out of the way.
1 Timothy 3:8-9
Deacons likewise must be men of dignity, not double-tongued, or addicted to much wine or fond of sordid gain, but holding to the mystery of the faith with a clear conscience.
Revelation 1:20
As for the mystery of the seven stars which you saw in My right hand, and the seven golden lampstands: the seven stars are the angels of the seven churches, and the seven lampstands are the seven churches.
Revelation 10:5-7
And the angel whom I saw standing on the sea and on the land raised his right hand to heaven and swore by him who lives forever and ever, who created heaven and what is in it, the earth and what is in it, and the sea and what is in it, that there would be no more delay, but that in the days of the trumpet call to be sounded by the seventh angel, the mystery of God would be fulfilled, just as he announced to his servants the prophets.
Revelation 17:4-7
The woman was arrayed in purple and scarlet, and adorned with gold and jewels and pearls, holding in her hand a golden cup full of abominations and the impurities of her sexual immorality. And on her forehead was written a name of mystery: “Babylon the great, mother of prostitutes and of earth’s abominations.” And I saw the woman, drunk with the blood of the saints, the blood of the martyrs of Jesus. But the angel said to me, “Why do you marvel? I will tell you the mystery of the woman, and of the beast with seven heads and ten horns that carries her.
The Matthew example is repeated in the other three gospels. There are 17 mentions of "mystery" in the passages above. Those 17 plus the three others in the other gospels makes for a total of 20 mentions. In all but one of them the "mystery" is said to have been a mystery in the past but made known in the New Testament era.
Therefore, be wary of teachers who say there is still a mystery remaining and they know what it is. They are not teaching in a manner consistent with the New Testament example.