How did Zoroaster's Gathas preserve early Iranian religious teachings?

The Gāthās—a collection of 16 or 17 short hymns—preserved early Iranian religious teachings primarily through oral tradition. As the oldest texts in the Avestan language, they are widely accepted as containing the original religious vision and teachings of the prophet Zoroaster.12

These hymns were composed in an archaic form of an Old Iranian language and were transmitted orally for centuries, as there is no evidence that religious compositions were reduced to writing in Iran until the late Arsacid or early Sasanian periods. Eventually, the Gāthās were embedded within the Yasna, a primary division of the Avesta, where they were preserved through liturgical recitation by priests during sacrificial ceremonies. By being integrated into this sacred collection, the Gāthās survived as a foundational record of Zoroaster’s reinterpretation of inherited Iranian religious ideas.123