The baptism of Croats was a gradual process that probably took place in the period from 7th to the 10th century, as evidenced by (different and inconsistent) records of the Byzantine emperor and chronicler Constantine VII. Porphyrogenitus. Most probably the first impetus for the baptism of Croats came from the Romanesque clergy in Dalmatia, which followed Rome’s efforts to restore the church hierarchy in the areas that perished under the invasions of the Slavs and Avars in 6th and 7th century, and especially in big Dalmatian cities. Also, the Byzantine authorities sought to carry out the Christianization and pacification of the immigrant pagan population. Given the political changes that followed and the supremacy of the Franks over Byzantium in that part of Europe, Frankish missionaries from the ecclesiastical centers of Aquileia and Salzburg worked among the Croats from the end of 8th century, mostly in the north and in Dalmatia, and Byzantine in the areas south of the river Cetina in Bosnia, Hum and Duklja.
The first reliable news about the first contacts of Croats with Christianity can be found in papal sources. Pope Gregory (590-604) recorded the arrival of the Croats. In 600 he wrote to Archbishop Maximus of Salona that he was “confused by the direct danger posed by the Slavic peoples, because they had begun to enter Italy through the Istrian crossing.” According to the “Liber pontificalis” Pope John IV. “A Dalmatian” (640–642) sent abbot Martin to redeem captured Christians and the bones of Christian martyrs held by Slavic pagans in Dalmatia. After passing through the lands occupied by the Croats, Martin returned to Rome with the bones of the Illyrian martyrs from the time of Diocletian’s persecutions (Anastasia, Dujam, Mauro, Venancius and others), and John IV had them stored in the oratory next to the baptistery of the Lateran Basilica in Rome. as evidenced by the large mosaic with their figures, erected by his successor Theodore I (642–649).
The news of the baptism of Croats by Thomas the Archdeacon in the chronicle “Historia Salonitana” connects with the activities of the first Archbishop of Split Ivan of Ravenna at the end of 7th century. According to him, the pope sent Ivan of Ravenna to Split to investigate the situation of Christians in Dalmatia, and after he submitted a report in Rome, the pope consecrated him archbishop of Split. Thomas describes his work in detail: he arranged the Church and the clergy of Split, transferred the bones of St. Dujam and Staša from Solin to the Split Cathedral, which he arranged in Diocletian’s mausoleum, restored churches, appointed bishops, deployed parishes and converted the population to Christianity. The credibility of the news about Ivan of Ravenna in historiography gradually became the key evidence for the dating and dynamics of the baptism of Croats.
An important moment for the baptism of all Slavs took place under the influence of the Byzantine emperor Michael III. (840 – 867). At the invitation of the Great Moravian prince Rastislav (846-870), he sent the missionary brothers Constantine (826/7-869) and Methodius (c. 815-885). According to “De administrando imperio”, the brothers began to preach Christianity among the Slavs, and they also introduced the Slavic language into worship and translated the Bible into the old Slavic language. They also edited the new Cyrillic and Glagolitic alphabets. The Cyrillic alphabet is still used as a script in many Eastern European countries, while the Glagolitic alphabet was mostly accepted by Croats, although it is no longer in use.
Another important contribution to the Christianization of Croats is given by Western Christianity. Franconia expanded its empire during the wars of Charlemagne (747-814). Its influence on Croatian territory was established after the Peace of Aachen in 812, when the border with Byzantium was determined, which ran right across the area of Southeast Europe. The largest and most dynamic baptizer of Croats was the Archdiocese of Salzburg, which sent missionaries to Carantania, Pannonian Croatia and Dalmatia.
Proof of the Christianization of the Croats are the letters exchanged by Pope John VIII. (ca. 820 – ca. 882) and Croatian dukes Domagoj, Zdeslav and Branimir from the second half of 9th century. Domagoj was warned in his letters for the conflict with the Venetians and piracy, and the pope asked him, like a true Christian ruler, to expel rather than kill political opponents. In his letters, he warned Zdeslav for his adherence to Byzantium. On the other hand, duke Branimir asked and received the recognition of Croatia from the pope after taking power, and in return he guaranteed fidelity to the pope.
The next step was the final ecclesiastical organization during the reign of King Tomislav (910–928), when the synods of Split (925 and 928), presided over by papal envoys, established the archdiocese in Split, and various religious and disciplinary arrangements were made. Papal domination and jurisdiction over the Dalmatian ecclesiastical territory was also established at that time, although it was probably not final even then, as traces of Byzantine influence were still felt for quite some time.