The Paḻayakūṟ (Pazhayakoor; Malayalam: പഴയകൂർ; English: "Old Allegiance"), also known as Malabar Chaldeans, Syrian Catholics of Malabar, Chaldean Syrians, Romo-Syrians or Syro-Malabarese, are the Saint Thomas Christians who use the East Syriac Rite and claim apostolic origin from the Indian mission of Thomas the Apostle in the 1st century AD.

The Saint Thomas Christians were originally in full communion with the Church of the East in Persia, from whom they inherited the East Syriac liturgical rite. Through the Schism of 1552, a faction of the Church of the East entered the Catholic Church. Following the 1599 Synod of Diamper, they were placed under the Latin Church's Padroado missionaries, who took over the jurisdiction of Angamaly. After the Coonan Cross Oath of 1653, which constituted a secession from the Padroado, the Paḻayakūṟ quickly returned to the Catholic Church as East Syriac Catholics under Archbishop Palliveettil Chandy.

Chandy's followers eventually became the Syro-Malabar Church, an Eastern Catholic Church in full communion with the Holy See. Another group within the Paḻayakūṟ returned to the traditions of the Church of the East and became the Chaldean Syrian Church, now part of the Assyrian Church of the East.

Origin Early history of Christianity in India Traditionally, Thomas the Apostle is credited for the establishment of Christianity in India. He is believed to have come to Muziris on the Malabar coast, which is in Kerala, in AD 52.

The Jewish community in India are known to have existed in Kerala in the 1st century AD, and it was possible for an Aramaic-speaking Jew, such as St. Thomas from Galilee, to make a trip to Kerala then. The earliest known source connecting the Apostle to India is the Acts of Thomas, likely written in the early 3rd century, perhaps in Edessa. The tradition of origin of the Christians in Kerala is found in a version of the Songs of Thomas or Thomma Parvam,"The Song of Thomas Ramban" in Menachery G (ed); (1998) "The Indian Church History Classics", Vol. I, The Nazranies, Ollur, 1998. in which, Thomas is described as arriving in or around Maliankara and founding Seven Churches, or Ezharapallikal: Kodungallur, Kottakavu, Palayoor, Kokkamangalam, Nilackal, Niranam and Kollam. Some other churches, namely Thiruvithamcode Arappally (a "half church"), Malayattoor and Aruvithura are often called Arappallikal. The Thomma Parvam further narrates St Thomas's mission in the rest of South India and his martyrdom at Mylapore in present-day Chennai, Tamil Nadu.

Church of the East in India An organized Christian presence in India dates to the arrival of East Syriac settlers and missionaries from Persia, members of what would become the Church of the East, in around the 3rd century. Saint Thomas Christians trace the further growth of their community to the arrival of Jewish-Christians from the region of Mesopotamia led by Knāi Thoma, which is said to have occurred in 345. However, most experts believe that the arrival of Knai Thoma must have occurred in the ninth century. The subgroup of the Saint Thomas Christians known as the Knanaya or Southists trace their lineage to Thomas of Cana, while the group known as the Northists claim descent from the early Christians evangelized by Thomas the Apostle. Byzantine traveller Cosmas Indicopleustes wrote of Syrian Christians he met in India and Sri Lanka in the 6th century. Even in Taprobané [Sri Lanka], an island in Further India, where the Indian sea is, there is a Church of Christians, with clergy and a body of believers, but I know not whether there be any Christians in the parts beyond it. In the country. called Malé [Malabar],' where the pepper grows, there is also a church, and at another place called Calliana there is moreover a bishop, who is appointed from Persia. Until the seventh century, the Saint Thomas Christians were included in the Metropolitanate of Persia. The metropolitan of Rev Ardashir, the head of the province, used to consecrated bishops for the Diocese of India. Patriarch Ishoyahb III (650-660) criticises Shemʿon, metropolitan of Rev Ardashir: "As far as your province is concerned, from the time you showed recalcitrance against ecclesiastical canons, the episcopal succession has been interrupted in India, and this country has since sat in darkness, far from the light of divine teaching by means of rightful bishops: not only India that extends from the borders of the Persian empire, to the country which is called Kaleh, which is a distance of one thousand and two hundred parasangs, but even your own Persia." The port at Kollam, then known as Quilon, was founded in 825 by Maruvān Sapir Iso, a Persian Christian merchant, with sanction from Ayyanadikal Thiruvadikal, the king of the independent Venad or the State of Quilon, a feudatory under Sthanu Ravi Varma Perumal of the Chera kingdom. Sapir Iso was the East Syriac Christian merchant who led the East Syriac bishops Mar Sabor and Mar Proth to the Christians of Malabar. The two bishops were instrumental in founding many Christian churches with Syrian liturgy along the Malabar coast and were venerated as Qandishangal (saints) since then by the Thomas Christians. It was during this period that Christians disappeared from the Coromandel Coast.

Chaldean metropolitans and the Portuguese Following the schism of 1552 in the Church of the East, when Monk Yohannan Sulaqa and his followers joined the Catholic Church, forming the Chaldean Catholic Church, both the traditional and the Chaldean Catholic factions sent bishops to India. The first of the Chaldean Catholic bishops in India was Yawsep Sulaqa, the brother of Yohannan Sulaqa. Another bishop, Abraham, arrived in India as a traditionalist bishop but later joined the Catholic faction.

Synod of Diamper and Coonan Cross Oath Abraham was to become the last Chaldean bishop to govern the undivided Saint Thomas Christian community. Following his death in 1597, the Portuguese missionaries, who had arrived along with the colonial traders to India, started a vigorous and comprehensive process of Latinisation in liturgy and discipline among the local Christians and prevented other East Syriac bishops from reaching Malabar. These efforts culminated in the so-called Synod of Diamper (1599), the local clergy was forced to reject the Chaldean Catholic patriarch of Babylon, who in fact was in full communion with Rome at that time, as a Nestorian heretic and schismatic. The Portuguese, who controlled the maritime routes to India at that time, continued to block the arrival of eastern bishops. They occupied the diocesan administration of the Saint Thomas Christians and deprived the archdeacon of his traditional rights. The wait of Syrian Christians for a Syrian bishop to restore their past ecclesiastical dignity and autonomy seemingly came to an end in the arrival of Ahattalla, a West Syriac bishop. But the Portuguese prevented Ahattalla from entering Malabar, despite the prayers of Archdeacon Thoma Parambil and the local Christians in 1653. This provoked a strong reaction from the local Christians, led by the archdeacon, in the form of the Coonan Cross Oath. Although the exact wording of the oath is disputed, its effect was the severing of the relationship between the local Christians and the Portuguese and the proclamation of the archdeacon as their new metropolitan with the title 'Mar Thoma'.

Schism Following the Synod of Diamper held in 1599 and organised by Aleixo de Menezes, the Primate of East Indies and Archbishop of Goa, many traditions and numerous Syriac books of the native Saint Thomas Christians were condemned. Although the Synod proposed a highly Latinised form of liturgy, it was resisted by most of them. Francisco Ros, the Latin Jesuit bishop who was appointed to the now downgraded bishopric of Angamaly, openly renounced the Synod of Diamper and called for a new synod at Angamaly to substitute it. This second Synod of Angamaly implemented a Latinised form of the Chaldean Rite among the Saint Thomas Christians. Being, a highly skilled Syriacist, this new Syriac liturgy was introduced by Roz himself. The text of the new liturgy largely consisted of translations from the Latin and intended to re-place the original East Syriac (Chaldean) rite of the local Christians.

Reunion with Rome In 1656, Pope sent an Italian 'Disclaced' Carmelite priest named Giuseppe Maria Sebastiani with the aim of bringing back the Saint Thomas Christians who had separated themselves from the jurisdiction of the existing Catholic bishop through the Coonan Cross Oath. In 1659, Sebastiani was appointed as Apostolic vicar of Malabar with the faculty of appointing a new bishop from the native Christians replacing the Portuguese bishop. He consecrated Chandy Parambil as the local bishop in 1663, after the Dutch, having defeated the Portuguese, banned other Europeans from operating in Malabar.

Liturgical change and schism With the presence of another local and validly appointed bishop, Thoma's authority increasingly began to be questioned and people abandoned him. With this, Thomas wrote letters seeking help from non-Eastern churches. In response, a Syriac Orthodox bishop named Gregorios Abdal Jalīl arrived in Malabar in 1665 and regularised Thoma's episcopacy. Succeeding Thoma, senior priests in his Pakalōmaṯṯam dynastic line took over as the leaders of the faction that remained aligned to him. They too maintained strong relations with the Syriac Orthodox Church. Over time, they adopted the West Syriac Rite instead of the old East Syriac Rite. Thus the split in the Saint Thomas Christian community solidified and those who descend from Thoma's faction came to be called ( or 'those of the new allegiance') and those of Chandy came to be called ( or 'those of the old allegiance').

The Puthankoottukar gradually shifted to West Syriac Rite and Miaphysitism. However, the majority among the Pazhayakoottukar also resisted the latinisation and a long struggle began for maintaining the Chaldaean rite and the contact with both the patriarchates of the Church of the East while identifying themselves Catholic in communion with the Holy See of Rome.

Propaganda rule Bishop Palliveettil Chandy tried to consecrate Thoma II, the leader of the Puthenkoor, as his successor, in an attempt to reunify both the Puthenkoor and the Pazhayakoor under a common Catholic hierarchy. However, this attempt was spoiled by the missionaries. Meanwhile, the missionaries assured Chandy that his successor will be a native. Hence, Chandy held his archdeacon as his rightful successor. But after Chandy's death, the missionaries appointed a half-Indian Portuguese, Raphael Figueredo () as the Vicar Apostolic of Malabar for Saint Thomas Christians. This appointment shook the confidence they had in the propaganda Carmelites and quarrels started to escalate. Many churches protested against the move and some even joined Thoma II. Soon, Bishop Raphael Figueredo also lost the favour of the Carmelites and he was replaced with Custodius de Pinho () as the Vicar Apostolic of Malabar. Chandy died in 1687 and with him the initial attempts for reunification of both factions also died out.

East Syriac attempts to Reunion Metropolitan Shemʿon and his mission in Malabar