Complex sentence: Saints Cyril and Methodius, Byzantine Greeks, born in Thessaloniki, were the creators of the first Slavic Glagolitic alphabet and Old Church Slavonic language, and also apostles-Christianizators of the Slavic world.
Split into simpler sentences: Saints Cyril and Methodius, Byzantine Greeks born in Thessaloniki, were the creators of the first Slavic Glagolitic alphabet and Old Church Slavonic language. They were also apostles-Christianizators of the Slavic world.