The territory of the present-day Archdiocese of Zagreb was part of the Roman province of Pannonia Savia, centered around the busy river port of Sisak. Christianity started to spread in Pannonia in the 3rd century. The capital of province, Sisak got its first bishop in the second half of the 3rd century. Bishop Castus was mentioned for the first time in 249 A.D. during Emperor Decius’s reign. One of the more notable bishops is Quirinus of Sescia, who suffered during the persecutions of Diocletian.[3] Later, the Councils of Split confirmed the Archbishopric of Split as the archepiscopal see having the right to govern all parishes on Croatian territory.[4]
In 1227 Pope Gregory IX confirmed the grants and privileges of the Zagreb Diocese, among which the most important, the Felitianus' Charter from 1134 A.D., the oldest preserved document of Croatian land between the rivers Sava and Drava.[5]Coloman, King of Hungary was crowned king of Croatia in Biograd na Moru in 1102.[6] Thus, the Diocese of Zagreb remained under the sponsorship of the King of Croatia and Hungary. The territory of the diocese changed several times throughout history.
On November 11, 1852, it was elevated to the status of an archdiocese.