The answer
The word "sacrament" or mystery when used in the Church is not used just in the way that the dictionary defines it, but is a technical term with a specific meaning of its own.
Each of the Church's mysteries or sacraments consists of a mysterious divine blessing which you cannot see, but which you receive in secret from the Holy Spirit through the prayers which a legitimate priest offers up in a special ritual, along
With the presence of a specific substance, which is the material of the sacrament or mystery.
It is not a mystery or sacrament in the sense of being something recognisable, such as when the Bible says: "the mystery of the seven stars" (Rev. 1:20)
A sacrament is conditional upon four elements which are: sacramental grace, a priest, prayers and ritual and the substance of the sacrament.
In baptism, for example, there exists something mysterious that is unseen, which is the new birth through water and the Spirit (John 3:5). Or you might say that in baptism you are being "clothed.. with Christ. " (Gal. 3:27), or that you "wash your sins away" (Acts 22:16) or that you are buried with Christ and die with Him (Rom. 6).
These heavenly blessings are a mysterious action which the Holy Spirit performs in the human being, through the priest's special prayers, and a special ritual which involves the one being baptised, being submerged in water three times. So here, then, the substance of the mystery is the water.
The mysterious blessing in the Chrism (the Myron) is the descent of the Holy Spirit, and in the sacrament of confession it is the wiping away of sins by the blood of Christ, and in the Eucharist it is the transformation of bread and wine into the body and blood of the Lord, while in marriage it is the joining together of the two into one etc. None of these blessings can be seen by the human eye, therefore they are a mystery.
They are things that cannot be distinguished by intellectual knowledge, like mysteries pertaining to ultimately knowable facts, data, learning or information, but are spiritual elements to do with faith which go beyond what can be expressed in words.
The Church has defined these mysterious blessings to be seven in number, and has prescribed special prayers for them and the rituals which they require.
There are, of course, other prayers and rituals which are not to do with the sacraments, such as the prayer for the departed, which isn't a sacrament but just simply a prayer, a request, in which the Church asks for compassion for the souls of those passing on.
And here "The knowledge of the secrets of the kingdom of heaven. " (Matt. 13:11), which are boundless, we now see as but "a poor reflection as in a mirror" (1 Cor. 13:12), but God will bring them to our knowledge in due time. These are not, however, part of the mysterious gifts which the believer receives on earth, and which the Church is engaged in giving to him by virtue of the authority granted to it by God.
So there is no need for anyone to confuse one kind of mystery with another.
Mysteries to do with things that are knowable, are quite different from mysteries in the sense of the sacraments which pertain to heavenly grace.
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Thrice-Blessed Pope Shenouda III of Blessed Memory