Abhayagiri (Sk.). Lit., “Mount Fearless” in Ceylon. It has an ancient Vihâra or Monastery in which the well-known Chinese traveller Fa-hien found 5,000 Buddhist priests and ascetics in the year 400 of our era, and a School called Abhayagiri Vâsinah,, “School of the Secret Forest”. This philosophical school was regarded as heretical, as the ascetics studied the doctrines of both the “greater” and the “smaller” vehicles— or the Mahâyâna and the Hinayâna systems and Triyâna or the three successive degrees of Yoga; just as a certain Brotherhood does now beyond the Himalayas. This proves that the “disciples of Kâtyâyana were and are as unsectarian as their humble admirers the Theosophists are now. (See “Sthâvirâh" School.) This was the most mystical of all the schools, and renowned for the number of Arhats it produced. The Brotherhood of Abhayagiri called themselves the disciples of Kâtyâyana, the favourite Chela of Gautama, the Buddha. Tradition says that owing to bigoted intolerance and persecution, they left Ceylon and passed beyond the Himalayas, where they have remained ever since.
Source: H.P.Blavatsky - The Theosophical Glossary