What sparked the spread of Christianity by the Greek Orthodox Church into Eastern Europe?

The spread of Christianity by the Greek Orthodox Church into Eastern Europe was significantly fueled by the missionary work of figures like Saints Cyril and Methodius in the 9th century. They created the Glagolitic alphabet, which facilitated the translation of The Bible and liturgical texts into the Slavic languages, making Christianity more accessible. Additionally, political alliances, such as the marriage of Byzantine princesses to Slavic leaders, helped establish Christianity as a vital part of cultural identity in these regions. The eventual establishment of local churches and the adoption of Orthodox practices solidified Christianity's presence in Eastern Europe.