When the Croats migrated in the 6th century from White Croatia (i.e. in the present-day region around Krakow in Poland), they brought with them, and subsequently developed , a palaeo-Croatian language, a branch of Palaeo-Slavonic. This language was divided into a number of dialects, among which the kaj dialect predominated in the North, the ca dialect in the South (Dalmatia) and part of Bosnia and the sto dialect in the South East. The dialect boundaries, however, cannot be clearly drawn. There were always areas overlapping and mixed dialects also existed.
At this early time in Croatian history, there was little linguistic differentiation in the Slavic world. The differences between the Croatian and the Czech dialects, for instance, were no greater than those that exist today between British and American English.
Although in the seventh century they were converted to Christianity by priests sent from Rome at the request of the Emperor Heraclius, in the ninth century, the Croats adopted as their liturgical language Old Church Slavonic, based on the Macedonian vernacular of Salonika, which they modified and adapted to their own use for secular puposes and which is today known as Croatian Slavonic or the Croatian version or "recension" of Old Church Slavonic. Two types of this language, liturgical used in religious services and secular used in non religious subjects, developed in the late Middle Ages. This religious and secular language, written in Glagolitic characters survived until the mid-19th century as the literary language of the Glagolitic clergy. This contrasted with the Roman clergy who used the Latin language in the Catholic liturgy. It was still used as a liturgical language in our own times and was replaced by contemporary literary Croatian after the second Vatican Council decision of 1965 that services could be held in the vernacular, and not, as previously, in Latin only. It is therefore this Croatian Slavonic which represents the first Croatian literary language. It became the vehicle of a substantial literature.
The first major text written in Croatian Slavonic is the Baska Tablet of 1100 recording the donation of a site by King Zvonimir to the Benedictine convent of the island Krk. This document, written in the Glagolitic script, was found in St. Lucy's church near Baska on the island of Krk. It stands as a cornerstone of Croatian literary development although fragments of earlier inscriptions written in the Glagolitic alphabet and dating from the eleventh century have been found on the islands of Krk and Cres (Valun Tablet) and in Istria (Plomin). Fragments of the oldest known Croatian Glagolitic manuscript - The Glagolita Clozianus (the beginning of the 11th century) - is a collection of sermons. The codex belonged to the dukes Frankopans from the island Krk.
Among the oldest documents in the Czech recension of the Old Church Slavonic written in Glagolitic are so called Kiev Folia named after the place where they are now kept (in the Academy of Science in Kiev). They consist of 17 parchment sheets possibly written in the 10th century in Moravia and contain part of a Greek missal translated into Old Church Slavonic. Parts of the same missal are also contained in the Vienna Folia which were written in Croatia about the end of the 11th century. In the opinion of experts, the Vienna folia are the oldest preserved Glagolitic documents with unmistakably Croatian features. There are two parchment sheets with the text in Old Church Slavonic. Under the text there are traces of an older text in Old Church Slavonic but unfortunately the older text of this Glagolitic palimpset cannot be read. The Vienna Folia are also the oldest testimony of the first contacts between Croatian Glagolitic and Czech mediaeval literature. These contacts developed particularly in the 14th century when Croatian Benedictines were invited by Charles IV of Bohemia to Emmaus Monastery in Prague (1347) to teach the Church Slavonic languages and Glagolitic script.
Compared to Old Church Slavonic, Croatian Slavonic is a Common Slavonic influenced by Croatian vernacular in the fields of phonetics, morphology and syntax, and, above all, vocabulary. This hybrid language which was semi-artificial in comparison with the language of the people, was used in notarial acts (in the Church), and in Glagolitic literature, which was, moreover, rich in an age when the number of literate Europeans was small. But every few texts written at that time had an exclusively aesthetic function. The great majority are of a religious character, liturgical or devotional.
After the schism between the Eastern (Orthodox) and Western (Roman) Christian churches in 1054 and the beginning of the Crusades, the Church Slavonic language fell out of use in all west Slavic countries. The only exception was the renaissance of Croatian Church Slavonic in the 13th century. In the late Middle Ages, within the framework of the Croatian State, an important technical literature appeared in judicial texts, charters and treaties, as well as a literature of translations of biblical stories, legends, apocrypha, hagiographies and Western mediaeval romances written in Croatian Slavonic. Among other works, the Chronicle of the Priest of Dioclea written about 1149, is outstanding.
It may be assumed that in parallel with this language there existed in the 14th and 15th centuries a popular literary language (vernacular) used in traditional oral folk poetry and in the poetry created by laymen for use in the afternoon office of the Church, the so-called "prose" for the divine worship. Unlettered people were not hindered in the creation of stories and fables, poetry, drama, or oratory. The first testimony of early Croatian dramtic performance, which is fully credited by scholars, tells of the reception of Pope Alexander III in Zadar in 1117, when the clergy and crowds of people gave recitations of chants in their native language (... cum immensis laudibus et canticis altisone resonantibus in eorum sclavica lingua...). But from the 12th century the national language, vernacular Croatian, was widely used for inscriptions, legal documents and digests of law, such as in 1189 Isprava Kulina Bana (the trade treaty between Dubrovnik and ban Kulin of Bosnia), 1275 Istarski Razvod (a record of a survey of the lands of Istria) and in 1288 Vinodolski Zakonik (the statue of Vinodol).
The poets who versified subjects for Church use seem to have been called zacinjavci "chanters". They, and others literary men, particularly translators, scribes, copyists and compilers, gradually introduced the vernacular into the literary field. They were the precursors of the rich poetic literature in the Croat language that appeared in the 15th and 16th centuries.
One of the oldest recorded Croatian poems is the 14th century Cantilena pro sabatho (1385) by an unknown author but transcribed by the Franciscan Paulus de Sclavonia (de Sebenico) who also recorded the Sibenik Prayer (Sibenska molitva), ca. 1347. Cantilena contans 132 evenly rhymed octosyllables and relates the events of Good Friday and Saturday. The poem is preserved in the 14th century Latin Codex kept in the Budapest library Szecheneyi.
With the appearance of the literary works destined for lay readers, resulting from the growing contact with Western literatures, the native vernacular gradually penetrated the literary language. The current daily language appeared more and more frequently in the translations of chivalresque romances like those of Troy (ca. 1300) and Alexander the Great, of spiritual lyrics such as the 15th century Torments of Jesus and , above all, in mediaeval mystery and miracle plays.