Initiation. From the same root as the Latin initia, which means the basic or first principles of any Science. The practice of initiation or admission into the sacred Mysteries, taught by the Hierophants and learned priests of the Temples, is one of the most ancient customs. This was practised in every old national religion. In Europe it was abolished with the fall of the last pagan temple. There exists at present but one kind of initiation known to the public, namely that into the Masonic rites. Masonry, however, has no more secrets to give out or conceal. In the palmy days of old, the Mysteries, according to the greatest Greek and Roman philosophers, were the most sacred of all solemnities as well as the most beneficent, and greatly promoted virtue. The Mysteries represented the passage from mortal life into finite death, and the experiences of the disembodied Spirit and Soul in the world of subjectivity. In our own day, as the secret is lost, the candidate passes through sundry meaningless ceremonies and is initiated into the solar allegory of Hiram Abiff, the “Widow’s Son”.
Source: H.P.Blavatsky - The Theosophical Glossary