In 1552 a section of the Church of the East, angered by Shemʿon VII Ishoʿyahb's misbehaviour, revolted against his authority. The prime movers in the rebellion were unnamed bishops of Erbil, Salmas and Adarbaigan, and they were supported by 'many' priests and monks from Baghdad, Kirkuk, Gazarta, Nisibis, Mardin, Amid, Hesna d'Kifa and Seert. These were urban centres where there was little respect for the principle of hereditary succession to the patriarchate. The rebels elected
Shimun Yohannan Sulaqa, the superior of the monastery of Rabban Hormizd near Alqosh, in opposition to Shemʿon VII Ishoʿyahb, but were unable to consecrate him as no bishop of metropolitan rank was available, as canonically required. Franciscan missionaries were already at work among the Nestorians, and they persuaded Sulaqa's supporters to legitimize their position by seeking Sulaqa's consecration by Pope
Julius III (1550–5). Sulaqa went to Rome, where he made a satisfactory Catholic profession of faith and presented a letter, drafted by his supporters in Mosul, which set out his claims to be recognized as patriarch. This letter, which has survived in the Vatican archives, grossly distorted the truth. The rebels claimed that the Nestorian patriarch Shemʿon VII Ishoʿyahb had died in 1551 and had been succeeded illegitimately by 'Shemʿon VIII Denha' (1551–8), a non-existent patriarch invented purely for the purpose of bolstering the legitimacy of
Shimun Yohannan Sulaqa's election. The Vatican was taken in by this fraud, and consecrated
Shimun Yohannan Sulaqa as 'patriarch of Mosul' and founding patriarch of the
Chaldean Catholic Church in April 1553 in Rome, thereby creating a permanent schism in the Church of the East. He returned to Mesopotamia towards the end of the same year. In December 1553 he obtained documents from the Ottoman authorities recognising him as an independent 'Chaldean' patriarch, and in 1554, during a stay of five months in Amid, consecrated five metropolitan bishops (for the dioceses of Gazarta, Hesna d'Kifa, Amid, Mardin and Seert). Shemʿon VII Ishoʿyahb responded by consecrating two more underage members of the patriarchal family as metropolitans for Nisibis and Gazarta. He also won over the governor of ʿAmadiya, who invited Sulaqa to ʿAmadiya, imprisoned him for four months, and put him to death in January 1555. The Vatican only discovered that Shemʿon VII Ishoʿyahb was still alive two years after Shimun VIII Yohannan Sulaqa's appointment. 12 January 1555, shortly after Shimun VIII Yohannan Sulaqa's murder, the Franciscan friar Ambrose Buttigeg wrote to Pope
Julius III with the news that 'Shemʿon Bar Mama' was still alive: :Your holiness will be shocked to learn that, contrary to what your holiness, the most reverend cardinals, and the rest of you were told, the old patriarch never died at all, and has recently murdered the said Simon Sulaqa. == Shemon's death and succession ==