Arab Christians faced significant persecution with the Muslim conquests of Christian countries after the death of the Prophet Muhammad. Conversion was often required for those who could not pay a substantial tax known as the 'jizya'. Furthermore, during various periods in the history of the Islamic world, Arab Christians were subject to at times brutal and humiliating persecution. Despite this, many Christians chose not to change their religion to Islam. Instead, they kept Christianity as their belief. Muslims call them the "People of the Book" (with Jews).