is however known that both Arbela and Kirkuk later served as important Christian centers in the Sasanian and later Islamic periods. According to some traditions, Christianity took hold in Assyria when Saint Thaddeus of Edessa converted King Abgar V of Osroene in the mid-1st century AD. From the 3rd century AD onward, it is clear that Christianity was becoming the major religion of the region, with Christ replacing the old Mesopotamian deities. Assyrians had by this time already intellectually contributed to Christian thought; in the 1st century AD, the Christian Assyrian writer Tatian composed the influential Diatessaron, a synoptic rendition of the gospels. Though Christianity is today an intrinsic part of Assyrian identity, Assyrian Christians have over the centuries splintered into a number of different Christian denominations. Though the prominent Assyrian Church of the East, the followers of which have often been termed "Nestorians", continues to exist, other prominent eastern churches include the Chaldean Catholic Church, which split off in the 16th century, the Syriac Orthodox Church, the Syriac Catholic Church, and the Ancient Church of the East, which branched off from the Assyrian Church of the East in 1968. Though these churches have been distinct for centuries, they still follow much of the same liturgical, spiritual and theological foundation. There are also Assyrian followers of various denominations of Protestantism, chiefly due to missions by American missionaries of the Presbyterian Church. Because the Assyrian Church of the East remains dismissed as "Nestorian" and heretical by many other branches of Christianity, it has not been admitted into the Middle East Council of Churches and it does not take part in the Joint International Commission for Theological Dialogue Between the Catholic Church and the Orthodox Church. This does not mean that efforts to approach ecumenism have not been undertaken. In 1994, Pope John Paul II and Patriarch Dinkha IV signed the Common Christological Declaration Between the Catholic Church and the Assyrian Church of the East, with some further efforts also having been made in the years since. Historically, the main obstacle in the way of ecumenism has been the ancient text Liturgy of Addai and Mari, used in the Assyrian churches, wherein the anaphora does not contain the Words of Institution, seen as indispensable by the Catholic Church. This obstacle was removed in 2001, when the Catholic Congregation for the Doctrine of the Faith determined that the text could be considered valid