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

The main merit of the two brothers Cyril and Methodius in the development of Russian culture was the creation of the first Slavic writing system in the Old Russian state: the Old Slavonic alphabet, the Church Slavonic language. These two people left a huge legacy behind them: the church calls them “equal to the apostles preachers” and “Slovenian teachers”, and the Russian Orthodox and Catholic churches honor these two people, and special church holidays are held in their honor in the spiritual world.