Brothers Cyril and Methodius were 9th-century Byzantine missionaries known for their work in spreading Christianity among the Slavic peoples. They created the Glagolitic alphabet, the first alphabet used to transcribe the Old Church Slavonic language, which helped in translating religious texts and promoting literacy. The brothers are often referred to as the "Apostles to the Slavs" due to their significant contributions to Slavic culture and education. Their efforts laid the foundation for the development of the Cyrillic alphabet, which is still used today in many Slavic languages, including Russian and Bulgarian.