Christianity spread to Mesopotamia and areas of the Persian Empire as early as the first Christian century. Many Chaldeans and Assyrians accepted the Gospel and gradually established "The Church of the East''. According to their tradition, St. Thomas the Apostle was the first to evangelize those regions in his journey to India, followed by Mar Addai, one of the Seventy Disciples of the Lord, and then by Mar Mari, his own disciple, both coming from the missionary base which was established in Edessa on the border of Syria and Mesopotamia. During the fourth and fifth century prominent centers of learning for the Church of the East were Edessa and Nisibis in Upper Mesopotamia. Early in the fourth century, the episcopal see of the Sasanid Capital Seleucia-Ctesiphon gained prominence among all episcopal sees of Mesopotamia and surrounding areas within the boundaries of the Persian Empire and soon became the see of the Catholicos (Patriarch) of the Church of the East. At the beginning of the seventh century, prior to the Islamic conquest of Mesopotamia (634 A.D.), almost one third of the population was Christian. Following the Islamic Conquest, Islam became the religion of the majority of the population. Christians and Jews were accepted in Islamic society as "the People of the Book" , and they were organized as religious, social and cultural communities under their own leaders and laws. During the patriarchate of Mar Timothee the Great (780-823), when the Arab Abbasides built Baghdad as the capital of their Empire, the Patriarchal See was transferred to Baghdad. The Abbasides turned to the Christian scholars of the country for the teaching and spreading of sciences and knowledge, especially in the fields of philosophy, medicine, chemistry, astronomy and mathematics. The Greek culture had been translated by the Chaldean scholars first to Chaldean-Aramaic, then to Arabic, and eventually reached the West via Spain. For four centuries ``The Church of the East'' considered itself part of the Catholic Church. In the fifth century, as the result of the Christological controversies, the majority of this Church adopted the Nestorian theological formulas, condemned in the Ephesian Council (431 A.D.), thus isolating itself from the Church of the Roman Empire, and was consequently called "the Nestorian Church''. In a millennium of isolation, the Church of the East accomplished the most prodigious and ambitious missionary expansion of the Middle Ages (between the 7th and the 13th century). "Nestorian" monks spread the gospel, together with the Aramaic alphabet and culture, among the peoples of Khurasan, Azerbaijan, Afghanistan, Soviet Turkistan, Mongolia, China, Tibet, India, Japan and the Philippines. The Stele of Si-Ngan-Fu in China (781 A.D.), and the 611 tombstones discovered in the province of Semiryenchensk in Southern Siberia (all inscribed in Chaldean) remain eloquent witnesses of the magnitude of Christian Chaldean expansion and influence. The living remnants of this missionary enterprise are the three million Indians in Malabar who still follow the liturgical rites of the Church of the East. The Mongolian persecutions in the first half of the 14th century were what decimated the children and most of the history of the Church of the East. At the beginning of the 15th century, good segments of the Church of the East, moved by the spirit of renewal, found the road of Rome again. Ecclesiastical unity was reestablished with the Catholic Church in 1553 through the efforts of Patriarch Shimun VIII Yohannan Sulaqa (1510-1555 A.D.). In January of 1555, upon his return from Rome, Patriarch Sulaqa was imprisoned, tortured and martyred by the local Ottoman authorities due to strong opposition by the opposing Patriarch. Till today, the Chaldean Catholic Church remains one of the twenty-three Eastern Rites (sui iuris) of the Catholic Church.