As Ustashi racialism had embarked upon a policy of Serbian extermination, it followed that its twin counterpart, Catholicism, could do no less than embark upon the extermination of its main religious foe: the Orthodox Church. State and Church, consequently, to implement their mutual scheme of total racial religious exclusiveness, set out to pursue parallel policies, epitomized in the extermination of the racial elements, the Serbs, by the political authorities, and in that of the religious elements, the Orthodox, by the Catholic Hierarchy.

The Catholic Church did not leave the execution of a religious war to the secular arm, as she had done in similar circumstances in bygone centuries. She came down into the fighting field, full tilt, shunning precautions and brandishing the sword against those whom she had decided to exterminate, with a directness that had not been seen for a long time. Many of the Ustashi formations were officered by Catholic priests, and often by friars, who had taken an oath to fight with dagger and gun for the "triumph of Christ and Croatia." Many of them did not hesitate to carry out the most infamous tasks, glorying in deeds that would have filled with shame any average "heathen or barbarian from the East." All in the name of religion. Thus, while some, as we have already seen, took charge of concentration camps, others led the armed Ustashi in the closing of Orthodox churches, in the confiscation of Orthodox records, in the persecution, arrest, and, yes, even in the murder of Orthodox people, including Orthodox priests. At Banja Luka, for instance, an official order directed that all the Orthodox Church records of marriages, baptisms, and burials be delivered forthwith to Catholic parishes, while at Pakrac Catholic priests took possession of the Serbian Bishop's residence, following the locking and sealing of the Orthodox cathedral (April 12, 1941).

Orthodox churches were converted into halls—e.g. that of Prnjavor, on July 10, 1941. Others were transformed into Catholic churches, when they were not pulled down altogether—e.g. in the provinces of Lika, Banija, and Kordun, where 172 churches were totally destroyed. Orthodox monasteries shared the same fate. At Fruska Gora fifteen Serbian Orthodox monasteries and churches were given to Catholic monks of the Franciscan order, as was also done with the Church properties at Orahovica, Pakrac, Lepavina, and other places. The monastery of Vrdnik-Ravanica, wherein were buried the remains of King Lazar, who led and died in the historical battle of Kosovo against the Turks in 1389 in defense of Christianity, was also taken over, as was Sremski Karlovci, the former seat of the Orthodox Patriarchate. There the great cathedral was first plundered of all valuables, then closed, after all its physical properties had been taken over by the Catholic Bishop. Within a short period 250 Orthodox churches were pillaged or destroyed. In the diocese of Diakovo, mentioned before, twenty-eight Orthodox churches became Catholic churches.
https://exploringrealhistory.blogspot.com/2019/01/part-3-vaticans-holocaustcatholic.html