The mass turn to Islam in the Central Rhodope Mountains happened between the 16th and the 17th century. According to the Codes of Bishop of Philippoupolis and the Czech historian and slavist Konstantin Josef Jireček in the middle of the 17th century, some Bulgarian provosts agreed to become Muslim en masse. They visited the Ottoman local administrator to announce their decision, but he sent them to the Greek bishop of Philippoupolis Gabriel (1636–1672). The bishop couldn't change their mind. According to the verbal tradition of the Greeks of Philippoupolis, a large ceremony of mass circumcision took place in front of the old mosque of the city, near the Government House. After that, the villagers became Muslim, too. According to the verbal tradition of the Bulgarians, Grand Vizier Köprülü Mehmed Pasha (1656–1661) threatened the Bulgarians of Chepino Valley that he would execute them if they didn't turn to Islam. In 1656, Ottoman military troops entered the Chepino valley and arrested the local Bulgarian provosts, in order to transfer them in the local Ottoman administrator. There, they converted to Islam. Grand Vizier Mehmed Köprülü, after the mass Islamization, destroyed 218 churches and 336 chapels in these areas. A lot of Bulgarians preferred to die instead of becoming Muslim. According to recent investigations the theory of forced conversion to Islam, supported by some scientists, has no solid grounds with all or most evidence being faked or misinterpreted. At the same time, the sincerity of the convert is a subject to suspicion and interrogation. Some authors for example, explain the mass conversions that occurred in the 17th century with the tenfold increase of the Jizya tax. Muslim communities prospered under the Ottoman Empire, as the Sultan was also the Caliph. Ottoman law did not recognize such notions as ethnicity or citizenship; thus, a Muslim of any ethnic background enjoyed precisely the same rights and privileges.