From BiblePortal Wikipedia

Charles Buck Theological Dictionary [1]

A term used to denote the secret rites of the Pagan superstition, which were carefully concealed from the knowledge of the vulgar. The learned bishop Warburton supposed that the mysteries of the Pagan religion were the invention of legislators and other great personages, whom fortune or their own merit had placed at the head of those civil societies which were formed in the earliest ages in different parts of the world. Mosheim was of opinion that the mysteries were entirely commemorative; that they were instituted with a view to preserve the remembrance of heroes and great men who had been deified in consideration of their martial exploits, useful inventions, public virtues, and especially in consequence of the benefits by them conferred on their contemporaries. Others, however, suppose that the mysteries were the offspring of bigotry and priestcraft, and that they originated in Egypt, the native land of idolatry. In that country the priesthood ruled predominant.

The kings were engrafted into their body before they could ascend the throne. They were possessed of a third part of all the land of Egypt. The sacerdotal function was confined to one tribe, and was transmitted unalienable from father to son. All the orientals, but more especially the Egyptians, delighted in mysterious and allegorical doctrines. Every maxim of morality, every tenet of theology, every dogma of philosophy, was wrapt up in a veil of allegory and mysticism. This propensity, no doubt, conspired with avarice and ambition to dispose them to a dark and mysterious system of religion. Besides the Egyptians were a gloomy race of men; they delighted in darkness and solitude. Their sacred rites were generally celebrated with melancholy airs, weeping, and lamentation. This gloomy and unsocial bias of mind must have stimulated them to a congenial mode of worship.

Webster's Dictionary [2]

(1): ( pl.) of Mystery

(2): ( pl.) of Mystery

The Nuttall Encyclopedia [3]

Sacred rites and ceremonies of stated observance among the Greeks and Romans in connection with the worship of particular divinities, to which only the initiated were admitted, and in which, by associating together, they quickened and confirmed each other in their faith and hope, and in which it would seem they made solemn avowal of these; the name is also applied to the Miracle Plays ( q. v .) of the Middle Ages.