"Religion of Holy Love," "Apostolic Christianity," "Doctrine of the Perfect..." This was the name of the Cathar faith, which became widespread in Europe in the 13th century - from Slavic lands to Provence and Languedoc in southern France. The story of this teaching, whose roots may be in the secret Gnostic cults of the East, is mysterious and tragic. The Cathar movement, formally destroyed by the Inquisition in the 13th century, became one of the foundations of the esoteric culture of Europe, the secret key to the poetry of the troubadours and the veneration of the Beautiful Lady, and maybe to some secret teachings of the Slavic world... All this is figuratively and with an unusually deep immersion in the topic tells the book of the French scientist Rene Nelly (1906-1982), first published in Russian, supplemented by no less rare texts that are printed in the Appendices.