What does « Hidden / Mystery » mean in the Quran?

Then after distress, He sent down upon you security [in the form of] drowsiness, overcoming a faction of you, while another faction worried about themselves, thinking of Allah other than the truth - the thought of ignorance, saying, "Is there anything for us [to have done] in this matter?" Say, "Indeed, the matter belongs completely to Allah." They conceal within themselves what they will not reveal to you. They say, "If there was anything we could have done in the matter, some of us would not have been killed right here." Say, "Even if you had been inside your houses, those decreed to be killed would have come out to their death beds." [It was] so that Allah might test what is in your breasts and purify what is in your hearts. And Allah is Knowing of that within the breasts.

They conceal [their evil intentions and deeds] from the people, but they cannot conceal [them] from Allah, and He is with them [in His knowledge] when they spend the night in such as He does not accept of speech. And ever is Allah, of what they do, encompassing.

If [instead] you show [some] good or conceal it or pardon an offense - indeed, Allah is ever Pardoning and Competent.

O People of the Scripture, there has come to you Our Messenger making clear to you much of what you used to conceal of the Scripture and overlooking much. There has come to you from Allah a light and a clear Book.

But what they concealed before has [now] appeared to them. And even if they were returned, they would return to that which they were forbidden; and indeed, they are liars.

Say, "Who rescues you from the darknesses of the land and sea [when] you call upon Him imploring [aloud] and privately, 'If He should save us from this [crisis], we will surely be among the thankful.' "

And they did not appraise Allah with true appraisal when they said, "Allah did not reveal to a human being anything." Say, "Who revealed the Scripture that Moses brought as light and guidance to the people? You [Jews] make it into pages, disclosing [some of] it and concealing much. And you were taught that which you knew not - neither you nor your fathers." Say, "Allah [revealed it]." Then leave them in their [empty] discourse, amusing themselves.

Call upon your Lord in humility and privately; indeed, He does not like transgressors.

Unquestionably, they the disbelievers turn away their breasts to hide themselves from Him. Unquestionably, [even] when they cover themselves in their clothing, Allah knows what they conceal and what they declare. Indeed, He is Knowing of that within the breasts.

It is the same [to Him] concerning you whether one conceals [his] speech or one publicizes it and whether one is hidden by night or conspicuous [among others] by day.

Our Lord, indeed You know what we conceal and what we declare, and nothing is hidden from Allah on the earth or in the heaven.

When he called to his Lord a private supplication.

And if you speak aloud - then indeed, He knows the secret and what is [even] more hidden.

Indeed, the Hour is coming - I almost conceal it - so that every soul may be recompensed according to that for which it strives.

And tell the believing women to reduce [some] of their vision and guard their private parts and not expose their adornment except that which [necessarily] appears thereof and to wrap [a portion of] their headcovers over their chests and not expose their adornment except to their husbands, their fathers, their husbands' fathers, their sons, their husbands' sons, their brothers, their brothers' sons, their sisters' sons, their women, that which their right hands possess, or those male attendants having no physical desire, or children who are not yet aware of the private aspects of women. And let them not stamp their feet to make known what they conceal of their adornment. And turn to Allah in repentance, all of you, O believers, that you might succeed.

[And] so they do not prostrate to Allah, who brings forth what is hidden within the heavens and the earth and knows what you conceal and what you declare -

And no soul knows what has been hidden for them of comfort for eyes as reward for what they used to do.

And [remember, O Muhammad], when you said to the one on whom Allah bestowed favor and you bestowed favor, "Keep your wife and fear Allah," while you concealed within yourself that which Allah is to disclose. And you feared the people, while Allah has more right that you fear Him. So when Zayd had no longer any need for her, We married her to you in order that there not be upon the believers any discomfort concerning the wives of their adopted sons when they no longer have need of them. And ever is the command of Allah accomplished.

Whether you reveal a thing or conceal it, indeed Allah is ever, of all things, Knowing.

The Day they come forth nothing concerning them will be concealed from Allah. To whom belongs [all] sovereignty this Day? To Allah, the One, the Prevailing.

He knows that which deceives the eyes and what the breasts conceal.

Indeed, those who inject deviation into Our verses are not concealed from Us. So, is he who is cast into the Fire better or he who comes secure on the Day of Resurrection? Do whatever you will; indeed, He is Seeing of what you do.

And you will see them being exposed to the Fire, humbled from humiliation, looking from [behind] a covert glance. And those who had believed will say, "Indeed, the [true] losers are the ones who lost themselves and their families on the Day of Resurrection. Unquestionably, the wrongdoers are in an enduring punishment."

O you who have believed, do not take My enemies and your enemies as allies, extending to them affection while they have disbelieved in what came to you of the truth, having driven out the Prophet and yourselves [only] because you believe in Allah, your Lord. If you have come out for jihad in My cause and seeking means to My approval, [take them not as friends]. You confide to them affection, but I am most knowing of what you have concealed and what you have declared. And whoever does it among you has certainly strayed from the soundness of the way.

That Day, you will be exhibited [for judgement]; not hidden among you is anything concealed.