Religion in Iran has been shaped by multiple religions and sects over the course of the country's history. Zoroastrianism was the main followed religion during the Achaemenid Empire (550–330 BC), the Parthian Empire (247 BC – 224 AD), and the Sasanian Empire (224–651 AD). In 651 AD, the Rashidun Caliphate conquered Persia and spread Islam as the main religion. Sunnism was the predominant form of Islam before the devastating Mongol conquest, but subsequently, Shi'ism became eventually utterly dominant in all of Iran with the advent of the Safavids. According to the 2011 Iranian census, 99.98% of Iranians believe in Islam, while the rest of the population believe in other officially recognized minority religions: Christianity, Judaism and Zoroastrianism. Because irreligion and some other rel