Eastern Iranian language of the Avesta , the sacred book of Zoroastrianism .
The oldest portion of the Avesta, the Gathas, are now commonly thought to date from around the end of the 2nd millennium BC and are thus contemporary with Vedic Sanskrit . Not until the mid- Sasanian period (5th6th century AD) was the Avesta committed to writing, in an alphabetic script invented for the purpose on the basis of existing Middle Persian scripts. The oldest manuscripts date only from the 13th century.