Yasna

[2] The function of the yasna ceremony is, very roughly described, to strengthen the orderly spiritual and material creations of Ahura Mazda against the assault of the destructive forces of Angra Mainyu.

These very ancient texts, in the very archaic and linguistically difficult Old Avestan language, include the four most sacred Zoroastrian prayers, and also 17 chapters consisting of the five Gathas, hymns that are considered to have been composed by Zoroaster himself.

The Encyclopedia Iranica summarizes the aim of the yasna ceremony as "the maintenance of the cosmic integrity of the good creation of Ahura Mazdā.

"[3] Zoroastrianism's cosmological/eschatological perception of the purpose of humankind is to strengthen the orderly spiritual and material creations of Mazda against the assault of the destructive forces of Angra Mainyu.

The inner chapters/sections (excepting chapters 42.1–4,52.5–8) are in the more archaic Old Avestan language, with the four sacred formulae bracketing the innermost core.

This innermost core includes the 17 chapters of the Gathas, the oldest and most sacred texts of the Zoroastrian canon.