Yasna 19 contains an Avestan exegesis of the Gathic stanza Ahuna Vairiia, the most revered text in Zoroastrianism. The stanza is traditionally understood to be the essential statement of the religion of Mazda. Thus, in Y 19 we have a unique opportunity to ask about the significance that the Gathas of Zarathustra held for the later Avestan tradition. In what intellectual horizon did Zoroastrian priests place their founding text? Although Y 19 exegesis of the Ahuna Vairiia contains semantic obscurities, it is possible to establish the meaning of the commentary through syntactic and conceptual analysis of two key terms and the phrases where they occur. The article critically examines the earlier interpretations of the text. Having found these inadequate, it proposes a new reading and understanding of the Avestan exegesis. In particular, the article argues that the Avestan exegete understood the Gathic stanza within an eschatological horizon.