Gradually there were increased number of laws regulating Zoroastrian behavior, limiting their ability to participate in society, and made life difficult for the Zoroastrians in the hope that they would convert to Islam. Over time, persecution of Zoroastrians became more common and widespread, and the number of believers decreased significantly. Many converted, some superficially, to escape the systematic abuse and discrimination by the law of the land. Others accepted Islam because their employment in industrial and artisan work would, according to Zoroastrian dogma, make them impure as their work involved defiling fire. According to Thomas Walker Arnold, Muslim missionaries did not encounter difficulty in explaining Islamic tenets to Zoroastrians, as there were many similarities between the faiths. According to Arnold, for the Persian, he would meet Ahura Mazda and Ahriman under the names of Allah and Iblis.