From Wikipedia
Cyril (born Constantine, 826–869) and Methodius (815–885) were two brothers and Byzantine Christian theologians and missionaries. For their work evangelizing the Slavs, they are known as the "Apostles to the Slavs".
They are credited with devising the Glagolitic alphabet, the first alphabet used to transcribe Old Church Slavonic.After their deaths, their pupils continued their missionary work among other Slavs.
The two brothers were born in Thessalonica (located in present-day Greece) – Cyril in about 827–828 and Methodius about 815–820. In 862, the brothers began the work which would give them their historical importance. That year Prince Rastislav of Great Moravia requested that Emperor Michael III and the Patriarch Photius send missionaries to evangelize his Slavic subjects.
 In 863, they began the task of translating the Bible into the language now known as Old Church Slavonic and travelled to Great Moravia to promote it.
The mission of Constantine and Methodius had great success among Slavs in part because they used the people's native language rather than Latin or Greek.

                                  The Czech Republic and Slovak Republic celebrate the holiday of Cyril and Methodius 5. July,

                                                                           as Roman Catholics in some countries.

This Cyril and Methodius mission is very important for history of Slavs in Central Europe thoug the inhabitants of this teritory were christianized already before.
Great moravian empire = Great Moravia, Latin Moravia Magna, the Greek Μεγάλη Μοραβία – megáli Moravía) was the first stable principality of western Slavs in central Europe. It covered  the territory of today's Moravia, Slovakia and Hungary between the years 833-907. Moravia is the eastern part of Czech Republic.
According to some sources and based on the fact that Constantin (Cyril) and Methodius (Metoděj) had knowledge of the Slavic language, it was judged that their mother was of Slavic origin.

                                                                                          Example of glagolitic alphabet

                                                                          
Glagolitic alphabet - is the Slavic font slightly older than the second Slavic font Cyrillic. Both were used for old Slavonic language. Glagolitic - the oldest Slavic Alphabet, was created before 863 by Constantine for missionary purposes in Great Moravia empire. It originated under the influence of Greek and Latin scriptures.However in the 10th century  the Slavs switched to Latin and Cyrillic, and the glaholics were no longer used.

                                                                                                  Example of Cyrillic alphabet

Statue of Cyril and Metoděj in Beskydy mountains                                             

                    Thanks for reading      

                                                                      Margaret