The Safavids made the greatest Iranian empire since the Islamic conquest of Persia , and established the IthnÄ Ë ashari ( twelver ) school of Shi ` a Islam as the official religion of their empire , making one of the most important turning points in the history of Iran . They ruled one of the greatest Persian empires since the Muslim conquest of Persia and established the Twelver school of Shi ` a Islam as the official religion of their empire , marking one of the most important turning points in Muslim history .