Patriarch John X of Constantinople

John X Kamateros (Greek: Ἰωάννης Καματηρός) was the Patriarch of Constantinople from 5 August 1198 to April/May 1206.

John was a member of the Kamateros family, to which belonged the Empress Euphrosyne Doukaina Kamatera, wife of Alexios III Angelos (r. 1195–1203). An educated man, well versed in classical literature, rhetoric and philosophy, he occupied a series of ecclesiastical posts reaching the post of chartophylax, which he held at the time of his elevation to the patriarchal throne.

In 1198–1200 he had an exchange of letters with Pope Innocent III on the issue of papal supremacy and the filioque clause. Notably, he disputed Rome's claim to primacy based on St Peter, and asserted that in reality its primacy came from the fact that Rome was the old imperial capital. He intervened in the riots in Constantinople against the arrest of the banker Kalomodios, and secured his release, but during the coup of John Komnenos the Fat on 31 July 1200, he hid in a cupboard as the rebels seized control of the Hagia Sophia.

John remained in office after Alexios III's deposition in July 1203, and according to Western sources, both he and Alexios IV Angelos, threatened by the Fourth Crusade, acknowledged papal supremacy in the same year. After the capture of Constantinople during the Fourth Crusade in 1204, he initially fled to Didymoteichon in Thrace. In 1206, Theodore I Laskaris invited him to Nicaea, where he had established the Empire of Nicaea, a Byzantine Greek successor state, but John refused, perhaps because of his advanced age, and died in April or May of the same year.

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