The Denkard, the Acts of the Religion, is a ninth or tenth-century Middle Persian compilation of which books three to nine survive, and it is the single most important source for reconstructing Zoroastrian doctrine. It contains systematic theology, ethical and legal material, a life of Zarathustra, and — critically for scholarship — chapter-by-chapter summaries of the twenty-one nasks of the Sasanian Avesta, most of which are lost, so that we know what was in books that no longer exist. It also contains extended polemic against Islam, Christianity, Judaism and Manichaeism, arguing among other things that a God who is the source of all things must be the source of evil too, which the Zoroastrian scheme avoids. The context matters: it was compiled by a community already living under Muslim rule, converting away, and conscious of decline, deliberately writing down as much as could be recovered. Much of what the world knows about Zoroastrianism comes from a salvage operation conducted by people who could see what was happening.