Ancient Iranian religion was strongly connected. Beginning in the early 10th-century BC, the ancient Iranian religion was gradually displaced by Zoroastrianism, which contains many essential aspects of its predecessor. The sources on ancient Iranian religion, though limited, consist of textual and material sources. The textual sources are both Iranian and non-Iranian. An important Iranian source is the Avesta, which are Zoroastrian sacred scriptures made in the Avestan. This is considered the principal source of knowledge on ancient Iranian religions. It is a collection of several texts that seems to have been over a large span of time by a variety of authors. These
Avesta collection now lies in the Royal Library, Denmark ('K'-series). Other large Avestan language manuscript collections are those of the British Museum ('L'-series), the K. R. Cama Oriental Library in Mumbai, the Meherji Rana library in Navsari, and at various university and national libraries in Europe. In its present form, the Avesta is a compilation from various sources, and its different parts date from different periods and vary widely in character. Only texts in the Avestan language are considered part of the Avesta. According to the "Denkard", the 21 "nask"s (books) mirror the structure of the 21-word-long "Ahuna Vairya" prayer: each