The Slavs were not the first to have liturgy in the vernacular. Constantine was well aware that Armenians, Syrians, Copts, and others were worshipping in their own languages, and well before his departure to Moravia he used this knowledge to justify the vernacular, preparing his defense of what the Latin papacy and the Franks and Germans saw as novelty. Besides being holy and zealous missionaries, not unlike their British and Irish predecessors, the greatest achievement of the apostles to the Slavs was that they formulated a new universal Christian theology of liturgy. It took a millennium until the Western Church finally acknowledged its importance.
With a few selected disciples of Slavic background, in 863 or 864 the brothers arrived in Moravia where they established schools modeled after the Byzantine system. The saints' mission, and especially their vernacular liturgy, was immediately condemned by the hostile German bishops as the "stupidities of the Greeks" After their three-year mission officially ended in 867, the brothers came to Rome, stopping in Venice where they had to face a congregation of hostile bishops, priests, and monks who attacked their liturgical innovations.
Why the Byzantine brothers ended up in Rome with the relics of St. Clement, rather than returning to Constantinople, is still unclear. But it is precisely this mysterious and controversial side trip which gives the whole mission its true meaning and significance. (9) Despite the popular myths and pseudohistories which strive to depict animosity between the papacy of Rome and the Byzantine empire, except for the occasional rivalry and excommunications, often just posturing, the relationship between the two ecclesiastical bodies was generally sympathetic. Many educated and intelligent popes, patriarchs, and emperors were trying to find the common ground not only for the political unification of Eastern and Western empires, but for the theological submission of the East to the supremacy of the Roman papacy, even if the political circumstances were quite often convoluted and hostile. By their coming to Rome and submitting to the power of the pope as the official Byzantine imperial emissaries, Saints Cyril and Methodius practically and effectively acknowledged the papal theological supremacy for all times, also validating the power of popes over kings, emperors, and bishops more effectively than the fraudulent Donation of Constantine, or the forgeries of the Frank monks known as the "Isidore Mercator" letters.
Pope St. Nicholas I the Great, the last of the three Great Popes, was the first pro-Slavic pope in history. He may have been elected with the approval of emperor Louis II who reigned in Italy, but the pope demonstrated that he was not the emperor's vassal and, as the pope, was also engaged in difficult and obscure negotiations with the various Byzantine parties, patriarchs, emperors, and Slavic rulers. While he certainly tried to promote and defend the papal supremacy, Nicholas also had the best intentions with respect to all the newly-converted pagan nations. It is quite possible that the brothers were invited to Rome by Pope Nicholas himself, perhaps even before their mission had begun, deliberately carrying the precious relics of St. Clement as an important symbol of Christian unity.
Nicholas did not see the fruit of his diplomacy and the Byzantine missionaries were kindly welcomed in Rome by newly-elected Pope Hadrian II, a pious priest who often prayed to the Blessed Virgin at the Basilica of St. Mary Major at the Crib (Ad Praesepse). It was no coincidence that the pope blessed the Slavonic liturgical books on the altar of St. Mary's, where the apostles, in the presence of the pontiff, in 867 chanted the first Slavic liturgy in Rome at Christmastime, Constantine being the only priest of the Slavic mission at that time.
The pope ordained Methodius to the priesthood in February 868 while placing the Slavonic Gospel on the altar of St. Peter's, and other disciples were soon ordained as well. The pope blessed the Slavic mission, asking the brothers to return to Moravia and Pannonia. In 868 another Byzantine embassy arrived in Rome, acknowledging that these Slavic lands belonged to the jurisdiction of Curia Romana.
While there is no direct evidence, only about eight old Slavic icons which depict Constantine and Methodius as equal bishops, it is not inconceivable that the pope had first consecrated Constantine as bishop. But the fragile Constantine fell ill and remained in Rome until his death in 869, Methodius alone carrying the burden. Before his death Constantine changed his name to Cyril, after the great theologian Cyril of Alexandria, the moving spirit of the third Ecumenical Council. (10)
At the request of Duke Kocel, the ruler of the Pannonian Slavs, the pope then consecrated Methodius in 869 as the titular bishop of Syrmium, and as the archbishop of the new Moravian-Pannonian archdiocese, making it more than clear that Methodius was the legitimate archbishop of all the the Slavs united in Rastislav's Great Moravia and in the Balkans. This daring plan was conceived by Pope Nicholas I, and it would have subordinated the Slavs directly to papal jurisdiction, balancing the disintegrating political power of the Franks and Germans, and possibly making Slavs the protectors of Italy, Rome, and of the papacy. How audacious the pope's plan was is well illustrated by what happened next.
During his return journey to Moravia in 870 the Bavarian bishops captured Methodius. He was treated badly, kept in cold and rain, and brutally attacked with a horsewhip by the angry Salzburg Bishop Hermanrich, who lost his rich Pannonian diocese and its income. After he was tried as heretic, Methodius was imprisoned in a secret place in Swabia. The papal nuncio in Germany, Anno of Freisingen, acted inexcusably when he rejected Methodius' appeals, and when he later lied to the pope denying that he even knew Methodius. Pope John VIII wrote strong letters to King Ludwig and to Hermanrich, and banned the celebration of the Holy Mass in Bavaria until Methodius was released in 873.
It was under these precarious circumstances that Svatopluk rose in prominence in 870 by betraying his uncle Rastislav to became a new German vassal ruler of Great Moravia. (11) The Peace of Forchheim in 874 negotiated by Pope John VIII was an after-math of Svatopluk's betrayal, designed to finally restore some peace in the troubled Moravia which was repeatedly attacked and plundered by the Germans. Nevertheless, Svatopluk's betrayal of Rastislav had likely sabotaged the great papal Byzantine Slavic vision for the world negotiated by Pope Nicholas I, and thus Svatopluk's seemingly inconsequential act of treachery may have had monumental consequences for Europe and for the world. It is Svatopluk who is celebrated today as the first great king of the Slavs. Later witnessing the destruction of his great Slavic empire, Svatopluk supposedly bitterly regretted his fateful decision, buried his sword, and died in a monastery as an unknown monk. The forgotten Rastislav was blinded and died imprisoned in a Bavarian monastery, becoming a saint in the Eastern orthodoxy.
The accusations against Methodius and the Byzantine liturgy continued, now coming also from Svatopluk. Methodius was called to Rome in 880 where he again successfully defended his theology, prompting the pope's letter to Svatopluk in which John VIII reaffirmed Methodius' mission. But a controversy still remains about a compromise which he later presumably...