The manuscript is an illuminated Gospel Book lectionary. The Ostromir Gospels contains the text and instructions for liturgy services on a given day. The book consists of those Gospel passages that are read during the Easter season. They are followed by Saturday and Sunday readings for the rest year. In the second part, services were arranged according to the church calendar. Readings "on different occasions" (for example, "on tsar's victory in the battle") are placed at the end of the book.
Many pages of the Ostromir Gospels include ekphonetic symbols added to the text. These musical signs helped the priest to chant a citation from the Bible. The musical recitation of the Gospels was used in the Greek Church, and such signs are mandatory in the Greek Gospel lectionaries of the 8th-14th centuries. They are extremely rare in Slavic manuscripts and are found only in two Slavic lectionaries: the Ostromir Gospels and the Kupriyanov (Novgorod) Leaves (OR RNB. F.п.I.58).
Since ekphonetic symbols are musical notations, so we can say that Russian sacred music originated in the Ostromir Gospels. There is strong connection between the evangelical text and the composition of Old Russian singing collections.
The church calendar of the Ostromir Gospels is of an enormous cultural and historical importance. It contains not only the feast days of the Eastern church, but also Western saints days. Such a mix of traditions suggests that the Ostromir Gospels, dating 1057, is perhaps the last surviving liturgical book reflecting the unity of the Christian church.As we know, the process of split of Christianity can be traced back to the mid-5th century, and ends with the "Great Schism" in 1054. The reason why the Ostromir Gospels so differs from the further Old Russian tradition, should be sought in the prototype manuscript from which the Ostromir Gospels was copied.
However, we can not ignore the historical realities: the broad dynastic relations between the Russian princely house and reigning families from other parts of the world. There are information on 38 members of the Russian ruling dynasty, who married foreign royal members in the 11th century. 8 intermarriages occurred in Germany, 2 - in France, 5 - in the Scandinavian kingdoms and in England, 7 - in Poland, 6 - in Hungary. 3 marriages were arranged with Polovtsian princesses, one with a Byzantine princess, 2 - with representatives of the Byzantine aristocracy. Thus, 27 marriages bound the Russian dynasty together with the Catholic West and only 3 marriages with Byzantium.
Is this what explain a unique mix of various traditions in the decoration of the Ostromir Gospels?