Definition of "mystery"

The case was that of a murder. It had an element of mystery about it, however, which was puzzling the authorities. A turban and loincloth soaked in blood had been found; also a staff.

1927, F. E. Penny, chapter 4, in Pulling the Strings

Then I had a good think on the subject of the hocussing of Cigarette, and I was reluctantly bound to admit that once again the man in the corner had found the only possible solution to the mystery.

1905, Baroness Emmuska Orczy, chapter 5, in The Hocussing of Cigarette‎

...and, not knowing the meaning or misterie of her pollicie, forgat no termes of reproche or rigorous rebuke against his chast doughter.

1567, Matteo Bandello, Certain Tragical Discourses of Bandello, tr. Geffraie Fenton

If God should please to reveal unto us this great mystery of the Trinity, or some other mysteries in our holy religion, we should not be able to understand them, unless he would bestow on us some new faculties of the mind.

1744 (first printed), Jonathan Swift, A Sermon on the Trinity

There are seven mysteries, or sacraments, in the Greek church, viz. baptism, the chrism (a rite peculiar to this church), the eucharist, confession, ordination, marriage, and the holy oil.

1809, Sir Robert Ker Porter, Travelling Sketches in Russia and Sweden: During the Years 1805, 1806, 1807, 1808

There’s no initiation either into such mysteries.

1899 February, Joseph Conrad, “The Heart of Darkness”, in Blackwood’s Edinburgh Magazine, volume CLXV, number M, New York, N.Y.: The Leonard Scott Publishing Company, […], part I, page 196

It is, indeed, part of the ritual of the candidate for adeptship into the British mysteries, resembling that for the neophyte into the Osirian, Cabiric or Orphean mysteries.

1928, Lewis Spence, Mysteries of Britain, page v. 123

The trades, the crafts, the mysteries, would all be losers.

1776, Adam Smith, An Inquiry into the Nature and Causes of the Wealth of Nations