Sacred mysteries are the areas of supernatural phenomena associated with a divinity or a religious belief and praxis. Sacred mysteries may be either:
Religious beliefs, rituals or practices which are kept secret from the uninitiated.
Beliefs of the religion which are public knowledge but cannot be easily explained by normal rational or scientific means.
A mystagogue or hierophant is a holder and teacher of secret knowledge in the former sense above, while mysticism may be defined as an area of philosophical or religious thought focusing on mysteries in the latter sense.
Greco-Roman mysteries
The mystery religions of antiquity were religious cults which required initiation of an "initiate" or new member before they were accepted, and sometimes had different levels of initiation, as well as doctrines which were mysteries in the sense of requiring supernatural explanation. In some, parts of the doctrine were apparently only known to priests. They included the Eleusinian Mysteries, Mithraism, the Cult of Isis, and the Cult of Sol Invictus. Mystery traditions were popular in ancient Greece and during the height of the Roman Empire, and parts of Early Christianity used secrecy in the same way.
Although the term is not used equally by all Christian traditions, many of the basic aspects of Christian theology require a supernatural explanation. To name but a few key examples, these include the nature of the Trinity, the virgin birth of Jesus, and the resurrection of Jesus. These are mysteries in the sense that they cannot be explained or apprehended by reason alone.
The word mysterion (μυστήριον) is used 27 times in the New Testament. It denotes not so much the meaning of the modern English term mystery, but rather something that is mystical. In the biblical Greek, the term refers to "that which awaits disclosure or interpretation". In the Catholic church the Latin term is mysterium fidei, "mystery of faith", defined in the Catechism of the Catholic Church (1997) to mean a mystery hidden in God, which can never be known unless revealed by God.