I. The Roman Age and the Middle Ages
On the territory of the present-day Archdiocese of Zagreb, in the Roman age was the province of Savska Pannonia (Pannonia Savia or Ripariensis). Christianity started to spread in Pannonia in the 3rd century.
The city of Sisak (municipium Siscia) was the residence of the Roman prefect, as well as of the army and river fleet commander. Being 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.
But the first historically important bishop of Sisak was St.Quirinus, a martyr at the time of Diocletian’s persecutions, in 303 A.D. According to Farlati, he was the bishop of Sisak for 33 years, as of 270 A.D. He was caught during religious persecution. Maximus, the head of the province, sent him to Subotište (Sabaria, present day Hungarian city Szombathely) to court. Bishop Quirinus refused to abjure his faith and yield to the Emperor’s divine image (ara Augusti) so he was sentenced to death and thrown in the River Sibar with a millstone around his neck.
The name of St. Quirinus stayed with the territory of his diocese. There is a village Kirin named after him, numerous churches in the area are consecrated to him and his name Kirin or Kvirin was a very common middle name in the area around the rivers Sava and Kupa until the 16th century, as official documents suggest.
The bishop of Sisak was subject to the Metropolitan archbishop residing at the former imperial capital, luxurious Mitrovica (Sirmium). His metropolitanate see territory was spread from Ljubljana (Emona) and Ptuj (Poetovia) in west up to Gigena (Oescus) in Bulgaria in east. In the 4th century, some bishops and parishes of the Srijem metropolitanate see embraced the Arian heresy, but the bishops in Sisak and the people stayed faithful to the Roman Catholic Church.
In 441 A.D., after the first fall of Mitrovica, Sisak joined the Metropolitanate See of Solin (Salona) so its bishops John and Constantine signed the documents of the First and Second Council of Solin in 530 A.D. and 533 A.D. During the turbulent times of the Avar and Slavic migrations in the 6th and 7th century, the Diocese of Sisak was abolished and Christianity in Pannonia started fading away.
After 400 years the diocese was nowhere to be mentioned. In 811 A.D. Emperor Charles the Great awarded the entire territory south of the river Drava to the patriarch of Aquileia. After the rebellion of Duke Ljudevit Posavski (810 A.D. – 823 A.D.) and his wars against the Franks, that imperial command was only a dead letter. Croatian Duke Trpimir (in 852 A.D.) believed that the territory of the Diocese of Sisak belonged to the Metropolitanate See of Split as legal successor to old Solin.
First attempt to renew the Diocese of Sisak was made in 925 A.D., at the First Council of Split during King Tomislav’s reign. Roman Catholic Dalmatian bishops wanted to resolve the question of Croatian bishop Grgur Ninski so they offered him one of the abolished dioceses, Sisak, Duvno or Skradin.
After some resistance, at the Second Council of Split in 928 A.D., Grgur chose Skradin as his episcopal seat so the Diocese of Sisak was not re-established. Meanwhile, the title ‘bishop of Croats’ was no longer used, but Toma, the archdeacon of Split, wrote in 1268 A.D., that its territory (territory of Pannonian Croatia and Slavonia) in the 11th century was still managed by the court’s ‘Croatian bishop’ (episcopus Croatensis).
Toma does not interpret the legal nature of the term ‘Croatian bishop’ but he said ‘he held a lot of parishes and land all over the Croatian kingdom..., and his jurisdiction stretched all the way to the River Drava.’ The residence of a ‘Croatian bishop’ is associated with the king's palace in Knin.
The disappearance of a ‘Croatian bishop’ from national and political history of the Croats coincides with the establishment of the Diocese of Zagreb, which at the end of the 11th century assumed the pastoral care of the areas of central and northern Croatia that once was under jurisdiction of a ‘Bishop of Croats’. In the south the newly established the Diocese of Zagreb borders with the diocese of Knin and the diocese of Krbava, established in 1185 A.D.