the church slavonic language
Church Slavonic is an Indo-European language still used as a liturgical language in some eastern orthodox churches. In days of yore it was written using the Glagolitic alphabet but later adopted a modified form of Cyrllic.
There are early biblical texts written in Church Slavonic, as well as many other documents. It's an interesting language with has some of the older features of Indo-European languages, such as a robust case system. For information about the alphabet, see here and here, and a PDF here. The pronunciation is here.