According to official science, Slavs got their first alphabet in 9th century. Some claim that Slavs were using writing system many centuries ago, but it’s not officially proved theory. Among Easter-Slavic nations there is a notion that Slavs used to write some sort of a runic-script on birch wood, similar to Vikings. On other hand there was few false-positive proof of Slavs writing such as notorious “Book of Veles”. In general a lot of theories mingle in this subject, so it would be best if we started easy and analyze the whole topic in several ways.
Proto-Slavic language is a name used for the reconstructed ancestor of all Slavic languages. It lasted till the 9th century. Some medieval sources mentioned writing system for it. One of them was Chernorizets Hrabar, the author of one literary work “On the Letters” (Old Church Slavonic: О писмєньхъ). He mentioned that the pre-Christian Slavs used strokes and incisions (Old Church Slavonic: чръты и рѣзы) to write.
Thietmar of Merseburg and Ahmad ibn Fadlan also wrote about Slavic scripts. It is thought that this may have been a form of runic script, but the writing system was not probably enough developed to reflect the spoken language. In Proto-Slavic verb to write derived from verbs with meaning to paint. It is probably remain of this proto-writing system. Some Slavic languages are still keeping this meaning. For example, in Serbian language verb to write has secondary meaning to paint.
Old Church Slavic
What we know for sure is that the Old Church Slavic was the first Slavic literary language written in glagolitic. The language was standardized by the mission of two brothers to Great Moravia in 863. Prince Rastislav of Great Moravia requested from Byzantine Emperor Michael III to send missionaries to evangelize his Slavic citizens.
Brothers Cyril and Methodius were familiar with Slavic language as they lived nearby Slavs. They began translating the Bible into Old Church Slavonic. It wasn’t an easy task to do as it’s known fact that by that time Bible had been only translated into three languages: Greek, Latin and Gothic. The Bible was written on Glagolitic alphabet which was created by the brothers. After finishing the translation, they traveled to Great Moravia to promote Christianity in language understandable for Slavs. In the Great Moravia despite some political difficulties, brothers enjoyed considerable success. After their death, they are venerated in the Orthodox Church as saints. Their day is celebrated by the Eastern Orthodox Church on the 11th of May as Saints Cyril and Methodius Day. In the Republic of Macedonia, May 24th is a national holiday known as the Saints Cyril and Methodius, Slavonic Enlighteners’ Day. In Bulgaria that day is known as Bulgarian Education and Culture and Slavonic Literature Day.
It was the first known alphabet used for Slavic manuscripts. It included 41 letters. Some graphemes were derived from medieval cursive Greek small alphabet, Hebrew alphabet, Coptic alphabet etc. Glagolitic letters were used as numbers also. It is named after Slavic word глаголъ (glagoliti – to speak). Interesting fact is that Glagolitic alphabet is the writing system used in the video game The Witcher.
Glagolitic lost its dominance in the 12th century under the influence of Cyrillic, which was invented in the First Bulgarian Empire in the Preslav Literary School by Clement of Ohrid, disciple of Cyril and Methodius. Only in Croatia the Glagolitic alphabet was used until the 19th century. Cyrillic was simplification of writing system for Old Church Slavic. It is derived from the Greek alphabet with adding some letters which was not found in Greek. It is named in honor of Cyril. The first known Cyrilic script is Samuil’s Inscription, found on the tombstone of the Bulgarian tsar Samuel’s parents dated in 923.
Corpus of Old Church Slavonic manuscripts was collected in canon. As earlier manuscripts are unknown, most of them are from the 10th and from the 11th century. Scripts must use language and tradition of Constantine and Methodius to be incorporated into the canon. They are classified in two groups depending on the alphabet used in the manuscripts.
Nowadays, Cyrilic is standard writing system in Slavic languages: Belarusian, Bulgarian, Macedonian, Russian, Rusyn, Serbian, Ukrainian. In other Slavic countries Latin alphabet is used.