The Croats lived north of the Carpathian Mountains before they moved to the South and settled in the present regions. This move took place during the siege of Constantinople by the Avaro-Persians in 626 and during the reign of Heraclius I the Byzantine emperor. He asked the Croats for help and invited them into Dalmatia. The Croats, after several years of bloody fighting, had defeated the Avars and pushed them to the north of the Danube. They then settled, as Constantine VII Porphyrogenitus writes, in Dalmatia, Pannonia and Illyricum which means: between the Adriatic Sea in the South and the Drava and Danube rivers in the North, and betwen the Rasa river in Istria in the West, the Drina river in the East and the Vojusa river in the Southeast which is present Albania.
When the Croats reached the shores of the Adriatic Sea, they were composed of seven clans which shared the same racial origin and spoke the same Croatian language. According to Slavic customs, these seven clans were jointly ruled by five brothers and two sisters; the highest authority was always invested in the eldest brother. The principal Croatian clan, namely, the clan that was under the immediate jurisdiction of the supreme governor of the Croats, had colonized the regions along the Adriatic Sea between the Cetina and the Zrmanja rivers, and inland up to the Vlasic and Borja Mountains. This indicates that the inland regions: Duvno, Hlivno, Glamoc, Grahovo and the entire Bosanska Krajina, except the Posavina, were inhabited by this principal Croatian clan. The rest of the clans had settled on the West, the North and on the East of this principal and strongest clan. One of these clans had colonized Zahumlje and Travunja. Hercegovina was known in the Middle Ages, by these two last names. The other clan had settled in Bosnia between the Vranica and the Vlasic Mountains in the West and the Drina river in the East.
The national and political unity of the Croats was upheld and preserved by the common language, Cakavstina; they were ruled by the same supreme governor; they had a joint national assembly which convened very often and they had a common military defense againt the outside enemy. In spite of all these common institutions which denote the national unity of the Croats, each clan had its own governor with its own autonomous government.
To prove that the Croats, when they had arrived in the South, had also taken Bosnia and Hercegovina as their permanent homeland, it is sufficient to analyze their social and aristocratic customs and practices. One will unmistakably discover that they are identical with those of the Croatian provinces, and at the same time different from those of the neighboring Serbs who live East of the Drina river. Since earliest times, Bosnia, as an autonomous unit, had for its governor the "Banus", and because of this name Bosnia was called Banus' Bosnia ( "Banovina Bosna" ). This title, "Banus", was specifically Croatian and was unknown either to the Serbs or, for that matter, to any other nation of Medieval Europe. This points out most evidently that Bosnia had ben colonized by the Croats since its earliest times. Who else would have used the purely Croatian title "Banus" for its governor and called Bosnia Banus' country (Banovina) but the Croats themselves.
The most ancient written documents testify that the Croats settled in Bosnia and lived there during the entire Middle Ages. An old Croatian chronicle, which originated in the eighth century, relates this about the first Croatian ruler: "And his kingdom was Bosnia and Valdemin (Valde-vino or Vinodol, near the present city of Rijeka) and extended to Polonija, (Polina, the old Apollonia near Valone ) his kingdom was bounded by the sea and extended inland".
The Croatian chronicle known by the name Methodos and written in 753, mentions that the provinces which composed the Croatian kingdom of that time were: White Croatia, Red Croatia and Bosnia. The authors of the chronicle, "Kingdom of Croats" (Kralievstvo Hrvata), written between 1074 and 1080, and also the chronicle of the Dioclean Priest (Chronica Presbyteris Diocleatis), written between 1149 and 1153, allege the same thing.
The Byzantine writer, John Cinnamus, describing the military expedition of Emmanuel Comnenus which had taken place in 1153. had this to say about Bosnia: "Bosnia is not subordinate to the governor of Serbia. She is entirely independent; her people have their own way of living and she governs herself". Therefore, it is obvious that the nation of Bosnia in the 12th century was different from that of the Serbs. The people of Bosnia in this era had their own autonomous government with its own specific customs and practices. That the nation of Bosnia could have been none other than the Croatian people, is proven by the fact that in Bosnia and in Rasa of neighboring Serbia from the eleventh century to our own day there were no other Slavic nations but the Serbs and the Croats.
When the Croats arrived in the South, they practiced Iranian-Slavic paganism At the request of Byzantium Pope John IV (640-642) Catholic missionaries were sent to the Croatian Provinces. These missionaries had converted the supreme governor of Croatia, Porga, and also a great many of the clan that was under his immediate authority, to the Catholic faith in 640. This had induced Pope John IV to renew the old archdiocese of Salona and transfer its see to the nearby city of Split (Spalatum). This pontiff also gave all the rights of the old archdiocese of Salona to the newly established archidiocese of Split. The newly founded archdiocese included in its jurisdiction all the districts of Croatia with present day Bosnia and Hercegovina and covered all the territory extending from the Adriatic Sea to the Danube River in the North, and to the Drina River in the East.
During the reign of the Bulgarian emperor Peter (927-969), a Bulgarian Orthodox priest known by the name, Bogomil, began to teach a new heresy which was called after him, namely, Bogomilism. The main characteristic of the new heresy was religious dualism, advocating a double principle: the principle of good and the principle of evil. This heresy had rapidly spread all over the neighboring countries, Croatia included. When Bosnia and Hercegovina had come under the dominion of the Bulgarian emperors between 990 and 1018, they had founded a separate diocese of Bogomilism in Bosnia for all the Croatian territories. This diocese had been known by the name of the Croatian Church, Ecclesia Sclavoniae. By the end of the 13 th century and at the beginning of the 14 th, the majority of the inhabitants of Bosnia and Hercegovina had become followers of Bogomil's heresy. After the failure of the Crusades of 1225 and those of 1235-1239, the Popes sent the Dominican Fathers, and a little later the Franciscan Fathers to Bosnia to bring the Bogomils back to the Catholic Church. Father Gerardus Eudes, General of the Franciscan Order, on the request of Pope Benedict XII (1334-1342) had established a special Franciscan unit, Vicaria Bosnae. From that time on, the Franciscan Order had been sending its best missionaries to Bosnia. This practice continued until Bosnia fell under the rule of the Turks. Many missionaries distinguished themselves as Bosnian Vicars: Among them are: Father Peregrinus of Saxonia, the first Bosnflan Vicar; Father John and Father Berengary, both of Aragonia (Spanish), Father Deodatus de Ruticinio and the well known musician, Father Nicholas ( Frelsch ), Father Andrew and Father Mathiaas, the latter was Bosnian Vicar 1411-1419 (English), Father Bartholomaeus of Alvernia, Vicar 1367-1407, Francis of Florence, Vicar 1350-1360, Father John of Padua, Father John Ristori St. Jacob of Marchia Vicar 1435-1439 Bl. Bernardine of Aquilla (Italian), Blessed Nicolas Tavelic, John Korculanski, Vicar 1432-1435, Francis and Philip of Dubrovnik (Croatian) and many others.
Due to the solicitous work of the numerous, capable and holy Franciscan missionaries, as Pope Boniface IX writes in his letter of March 7, 1402, 500.000 followers of Bogomilism had been brought back to the Catholic Church ("quingenta milia personarum infidelium"). Just prior to the conquest of Bosnia by the Turks, the population of Bosnia and Hercegovina was as follows: 800,000 Croatian Catholics; Croats of the Bogomilism heresy between 80,000 & 90.000: Non-Slavic Vlachs between 12,000 and 15,000; and the Serbs between 10,000 and 15,000. The Serbs lived in Podrinje which had been taken away from the old Serbia by the Bosnian ruler Tvrtko I and annexed to Bosnia between 1363-1366.