Saints Cyril and Methodius

In 846 prince Rastislav reigned in Morava (Czech Republic, Slovakia, Hungary). First, German missionaries from the kingdom of Franks should convert his people, but Rastislav feared the German influence. He wanted to erect an independent empire. Therefore he consulted the Byzantine emperor Michael III. The emperor sent him two monks, the brothers Constantine and Methodius. They came from Thessalia (Greece) which was under Slavic rule at this time. Constantine was very talented in eastern languages.

The Byzantine king was very proud of their achievements in the empire of Khan Hazarius where they convinced the people to convert to the Christian religion. So he immediately sent them to the empire of prince Rastislav.

The Slavic missionairies Cyrill and Methodius.
(picture from the subterranean basilica of St. Clement in Rome)

Pope Adrian II solemnly approved use of Slavonic language in the church services and commended their work. He then consecrated Methodius bishop.

They did a good job in teaching Christian faith in Slavic languages. But many believers from the Frank empire had the opinion that only 3 languages, Hebrew, Greek and Latin, were appropriate for the service in God. The conflict escalated so far that Pope Nicolas I. had to call for them. He welcomed them very kindly (868). The monks showed him their bible translations. Since then their work was officially approved. But shortly afterwards Constantine, who has been named Cyrill since he had become a monk, died in Rome. The Pope appointed Methodius as new head of the archbishopric of Pannonia and Srijem. 

More on Wiki:

http://en.wikipedia.org/wiki/Saints_Cyril_and_Methodius

Svätý Cyril a Metod, vy ste vyviedli našich predkov z temnoty pohanstva, priviedli ste ich k pravej viere v Ježiša Krista a teraz sa s nimi radujete vo večnej sláve. Oslavujeme Boha, že tak bohato odmenil vašu neúnavnú horlivosť v ohlasovaní evanjelia. Pokorne ďakujeme nebeskému Otcovi, že vašou apoštolskou činnosťou začlenil náš národ do svojej Cirkvi. Orodujte u Boha za slovenský národ. Vyprosujte nám silu, aby sme vždy statočne vyznávali svoju vieru a podľa nej aj žili.Orodujte u Boha za celú Európu, ktorej ste nebeskými ochrancami. Pomáhajte dnešným generáciám európskeho kontinentu udržiavať a zveľaďovať duchovné a kultúrne dedičstvo, ktoré dostali ich predkovia prijatím kresťanstva.Orodujte za cirkevné spoločenstvá Východu i Západu, aby odstránili spomedzi seba všetko, čo ich rozdeľuje, a pod ochranou Bohorodičky Panny Márie nažívali v plnej jednote a láske na stále väčšiu slávu Trojjediného Boha.