Yahballaha III (c. 1245–13 November 1317), known in earlier years as Rabban Marcos (or Markos) was Patriarch of the East from 1281 to 1317.

As patriarch, Yahballaha headed the Church of the East during the severe persecutions under the reign of khans Ghazan and his successor Öljaitü.

[1] A native of Koshang, Marcos travelled with Rabban Bar Sauma, an ascetic Nestorian monk from Mongol-controlled China to Jerusalem.

Patriarch Denha I of the Church of the East recalled them and consecrated Markos as the bishop of Katay and Ong, Mar Yahballaha.

In 1289, Yahballaha allowed the Dominican friar Riccoldo da Monte di Croce to preach among the Nestorians and renounced their heterodoxies.

Markos was born in the city of Koshang (near modern Dongsheng District, Inner Mongolia) the capital of the Turkic Ongud tribe.

The Arabic Chronicle of the Nestorian Patriarch calls him "a Turk by birth from the region of Katay (i.e. Northern China)".

After staying in Hotan for six months and finding Kashgar empty as its population fled the "enemy", Bar Sauma and Markos went to Taraz (north of Tien Shan) in present-day Kazakhstan to pay homage to Kaidu Khan and ask for safe passage through his land, which he allowed.

[6] The two travelers probably passed through Samarkand and Bukhara, arriving in the region of Khorosan in the town of Tus, now a village near Mashhad in present-day Iran.

[4] Their plan to visit Jerusalem was prevented because of the war between the Mongols and Mamluks, who at the time bordered each other along the Euphrates river.

At the same time, he named Bar Sauma sa'ora (visiting bishop) for the Eastern countries,[7] and general vicar.

[9] After the death of Patriarch Denha I, the Nestorian bishops chose Yahballaha as his successor in November 1281, with approval from Abaqa Khan, the Mongol ruler of the Ilkhanate.

Tekuder became unpopular among the Mongol elites,[12] the so-called "Old-Mongol" party of Nestorian Christians and Buddhists,[13] who now favoured his nephew Arghun, Abaqa's son.

[15] He wrote to Pope Honorius IV that Kublai Khan commissioned him to liberate the "land of the Christians".

From Genoa, Bar Sauma went to Lombardy and onwards to Paris, where he met Philip IV of France.

Before returning to Rome, Bar Sauma spent the winter in Genoa and met Pope Nicholas IV after his election.

[15][16] The Pope issued a bull recognising Yahballaha as the "patriarch of all the Christians of the East"[1] Bar Sauma returned to Ilkhanate in the summer of 1288.

[17] Dominican Friar Riccoldo da Monte di Croce travelled to the East in 1289 and remained there for ten years.

[19] However, relations remained good between the Church of the East and the new khan Gaykhatu, who succeeded Arghun that year.

[21] We believe in the holy Roman chief pontiff and universal father of all believers in Christ, and confess that he is the successor of the blessed Peter, universal vicar of Jesus Christ over all the sons of the church from east and west; love and affection for whom is fixed in our hearts; and we owe obedience to him, and ask and implore his blessing, and are ready for all his commands, humbly asking and imploring his help in our necessity and tribulations in which we have now been for a long time and remain.

And may the good father not turn away his face from us since we are all brothers in Christ and his sons through the true catholic faith.