Is Mormonism a Mystery Religion? Given some of the comments in this question, I feel compelled to ask: "Does Mormonism constitute a present-day example of a mystery religion?" I'm interested in both insider and outsider perspectives.
The article states that Christianity itself, particularly as it originally existed in the days of the early Church, meets the definition of a mystery religion, and that this was noted and remarked upon by one of the great early apologists, Justin Martyr.  And if the definition is that "participation is reserved to initiates," then Judaism meets the definition as well: Gentiles who had not converted could not take part in many of its sacred rituals, particularly ones that involved the Temple.  The Temple had an outer area known as the court of the Gentiles where outsiders could come, but beyond that, they were not welcome.

The Church of Jesus Christ of Latter-Day Saints claims to be the restoration of original Christianity, so it's not surprising to see such features as part of its theology.  Participation in sacred temple ordinances is "reserved to initiates," as the article describes it, and certain matters regarding temple ceremonies are not to be discussed outside of the temple.

However, there is an important distinction to understand:  to Latter-Day Saints, these matters are not secret; they are sacred.  I explained the difference in this question: the value of a secret is in exclusivity, whereas Latter-Day Saints actively invite and work to persuade all to come to the temple, but only once certain standards of preparedness, understanding, and spiritual maturity have been met.