Mithras, pagan beliefs, and Zoroastrianism were commonly practiced in Georgia. Christianity gradually replaced all the former religions except Zoroastrianism, which became somewhat of a second established religion in Iberia after the Peace of Acilisene in 378, which placed Georgians permanently on the front line of conflict between the Islamic and Christian worlds. Georgians remained mostly Christian despite repeated invasions by Muslim powers, and long episodes of foreign domination. After Georgia was annexed by the Russian Empire, the Russian Orthodox Church took over the Georgian church in 1811. The Georgian church regained its autocephaly only when Russian rule ended in 1917. The Soviet regime that ruled Georgia from 1921 did not consider revitalization of the Georgian church an important goal, however. Soviet rule brought severe purges of the Georgian church hierarchy and frequent repression of Orthodox worship. As elsewhere in the Soviet Union, many churches were destroyed or converted into secular buildings. This history of repression encouraged the incorporation of religious identity into the strong nationalist movement and the quest of Georgians for religious expression outside the official, government-controlled church. In the late 1960s and early 1970s, opposition leaders, especially Zviad Gamsakhurdia, criticized corruption in the church hierarchy. After Ilia II became the patriarch (catholicos) of the Georgian Orthodox Church in the late 1970s, Georgian Orthodoxy experienced a revival. In 1988 Moscow permitted the patriarch to begin consecrating and reopening closed churches, and a large-scale restoration process began. The Georgian Orthodox Church has regained much power and full independence from the state since the restoration of Georgia's independence in 1991. It is not a state religion, but its special status is recognized by the Concordat of 2002. Apart from the Georgian Orthodox Church, Christianity in Georgia is represented by followers of the Armenian Apostolic Church and the Russian Orthodox Church, and a Georgian Catholic Church which mostly follows either the Latin Rite or the Armenian rite. A 2015 study estimates some 1,300 Christian believers from a Muslim background in the country, most of them belonging to some form of Protestantism. Islam Islam in Georgia was introduced in 645 AD during the reign of third Caliph of Islam, Uthman. During this period, Tbilisi (al-Tefelis) grew into a center of trade between the Islamic world and northern Europe. Islam's history continued in Georgia throughout the late 14th and early 15th centuries with Timur's invasions of Georgia and during the 16th and early 19th