What was the merit of Cyril and Methodius in the development of Slavic culture?

Cyril and Methodius became the founders of the Slavic alphabet – Glagolitic, which later became known as Cyrillic. To do this, they had to carry out a gigantic job of isolating all the basic sounds of the Slavic language. There are discrepancies in different sources. Some indicate the authors of the alphabet of Cyril and Methodius, others indicate that the alphabet is the fruit of the work of the students of Cyril and Methodius, who based on the work of their teachers (on isolating the sounds of the Slavic language) created the alphabet. The creation of the Glagolitic alphabet made it possible to translate church books into the Slavic language, which made it possible for the Slavs to serve services in churches in their native language and organize a Slavic church. This contributed to the preservation of Slavic culture and identity.