Formed in 1985 from the Diocese of Kochi, Kunnamkulam holds Arthat St. Mary's, the oldest church in Kerala still Orthodox, and produced Catholicos Baselios Marthoma Paulose II.
The Christian community of Kunnamkulam and the historic Chattukulangara (Arthat) Church trace their origins to the first-century mission of St. Thomas the Apostle. Arthat St. Mary’s Orthodox Cathedral is the oldest church in Kerala that remains Orthodox. Throughout its history Kunnamkulam has stood as an island of Orthodoxy, ringed by Catholic communities on one side and, to the north, by the largely non-Christian lands of Malabar, and it endured repeated attacks and massacres in the time of Tipu Sultan. The diocese is also the home of the Twenty Martyrs of Arthat, nineteen laymen and one priest, who were martyred by the army of Tipu Sultan for refusing to convert to Islam.
Among the sacred sites of the diocese, the Pazhanji Church holds a distinguished place as the Second Kabaridam (Randam Kabaridam in Malayalam), a shrine of the relics of St. Yeldho Mar Baselios, whose principal tomb lies at Kothamangalam Cheriapally. It is related that Cheruvathoor Variyathu, while on pilgrimage to Kothamangalam Cheriapally, where it was then the custom to gather a little soil from the saint’s holy sepulchre, brought home a handful of that soil, unaware of the sanctity of its contents, and set it aside in a neglected corner of his house. Many troubles thereafter beset his family. Perceiving that these arose from the disrespect shown to the holy relics, Cheruvathoor Variyathu, in deep contrition, made his way on his knees from Pengamukku to the Pazhanji Church, where the relics of St. Yeldho Mar Baselios were enshrined. Two altars were afterwards constructed in the madbaha, and the holy relics were reinstalled in the southern thronos.
The saint is revered throughout the diocese as Pazhanji Muthappan, and thousands of the faithful come on pilgrimage to the church.
The diocese was established on 1 August 1985 from the Diocese of Kochi under Baselios Marthoma Mathews I. Paulose Mar Milithios was appointed first Metropolitan that day and was raised formally on 25 October 1991. The diocese grew rapidly under him, with Arthat St. Mary's Cathedral as its spiritual hub and the Bishop's House at Arthat as headquarters. In 2010 he was enthroned as Baselios Marthoma Paulose II, Catholicos of the East and Malankara Metropolitan. After his death in 2021, Geevarghese Mar Yulios became Diocesan Metropolitan, overseeing dozens of parishes, educational bodies and charitable ministries.
4 districts, listed alphabetically.