gathic

After the Gathas proper we have other hymns written in the Gathic dialect, from which the history of the religion after its foundation may be to some extent inferred. These show that the Zarathustrian religion was regarded, after the departure of the founder, as a great divine institution, and was worked out on the lines he had laid down. The forms of it became of course more fixed.

Allan Menzies - History of Religion: A Sketch of Primitive Religious Beliefs and Practices, and of the Origin and Character of the Great Systems