Cyril and Methodius
From Slavic.info
(Greek: Κύριλλος καὶ Μεθόδιος, Old Church Slavonic: Кѷриллъ и Меѳодїи)
Cyril is credited with having invented or adapted, a special alphabet which now bears his name in order to express the sounds of the Slavic dialect, as spoken by the Bulgarians and Moravians of his day. Later on Methodius translated the entire Bible into Slavic and his disciples afterwards added other works of the Greek saints and the canon law.
The two brothers always celebrated Mass and administered the sacraments in the Slavic dialect. News of their successful missionary work among the pagan Slavs was carried to Rome along with complaints against them for celebrating the rites of the Church in the "heathen" vernacular. In 868 both brothers were summoned to Rome by the Pope Nickolas the First, but arriving there after his death they were heartily received by his successor, the Pope Adrian the Second, who approved of their Slavic version of the liturgy. Cyril died in Rome in 896, and is buried in the church of San Clemente. Methodius was afterwards consecrated Archbishop of Moravia and Pannonia and returned thither to his missionary work. Later on Methodius was again accused of using the "heathen" Slavic tongue in the celebration of the Mass and in the sacraments. It was a popular belief then, that as there had been three languages (Hebrew, Greek, and Latin) inscribed over Jesus Christ on the Cross, it would be sacrilegious to use any other tongue in the service of the Church. Methodius appealed to the Pope and in 879 he was again summoned to Rome, before Pope John, who, after hearing the matter, sanctioned the use of the Slavic dialect in the Mass and the offices of the Church, saying among other things this:
"We rightly praise the Slavic letters invented by Cyril, in which praises to God are set forth, and we order that the glories and deeds of Christ our Lord be told in that same language. Nor is it in any wise opposed to wholesome doctrine and faith, to say Mass in that same Slavic tongue (Nee sanae fidei rd doctrinae aliguid obstat miss am in eadem slavonica langua canere), or to chant the holy gospels or divine lessons from the Old and New Testament duly translated and interpreted therein, or the other parts of the divine office: for He who created the three principal languages, Hebrew, Greek, and Latin, also made the others for His praise and glory."
From that date onward the Slavic language was firmly fixed as a liturgical tongue of the Church, and was used wherever the Slavic peoples were converted to Christianity under the influence of monks and missionaries of the Greek (Eastern) Church. The great success of the first Slavic Apostles was met with great enthusiasm in Moravia and Slavonia, but the jealousy of the Latin Roman communion was aroused, and, although their version of the Bible and liturgy was fully recognized by the Latin Roman Church, their followers were not able to stand against the opposition of the German prelates and priests.
No doubt it was a German intrigue that caused the failure of the great scheme of Methodius: to give the Slavs a national liturgy, in conjuction with Rome; to make of them a third branch of the Universal Church. Owing to this failure, the Slavic world found itself condemned to religious dismemberment, to that division between Latin and Orthodox which has been one of its misfortunes.