Hydranos (Gr.). Lit., the “Baptist”. A name of the ancient Hierophant of the Mysteries who made the candidate pass through the “trial by water”, wherein he was plunged thrice. This was his baptism by the Holy Spirit which moves on the waters of Space. Paul refers to St. John as Hydranos, the Baptist. The Christian Church took this rite from the ritualism of the Eleusinian and other Mysteries.
Source: H.P.Blavatsky - The Theosophical Glossary