Malayalam, English and Tamil are the spoken languages in the diocese.
The diocese of Kothamangalam was established in the year 1956, and inaugurated in January, 1957. Mar Mathew Pothanamuzhy was appointed as the first bishop of the diocese of Kothamangalam in 1957. On his retirement in 1977, Mar George Punnakkottil was appointed the bishop. The Kothamangalam diocese initially encompassed the areas of the current Idukki diocese also, until the new Idukki diocese was formed in 2002. The diocesan cathedral is St George's Cathedral, located at the High Range Junction, the centre of Kothamangalam town. The eparchy covers 12 foranes, with 115 parishes. Recently the diocese was bifurcated to form Idukki diocese. Kothamangalam is a small town lying on the foothills of the Western Ghats and is referred to as The Gateway to the High Ranges. Until the recent past, the town was also a very important trading center for spices and hill produce. Kothamangalam is known for its old Christian churches and its prominent educational institutions. St. Thomas is believed to have preached in this place and converted about 240 Brahmin families to the Christian faith.
The diocese enjoys four seasons - Winter, Summer, South-West Monsoon and North-East Monsoon. The winter season starts with the end of the northeast monsoons i.e. from the later part of November lasting till the middle of February. During this season temperature is comparatively low. In the Highlands where the climate is cool throughout the year, winter temperatures often fall below 10C. This season witnesses the lowest amount of rainfall. The flora is tropical. The heavy rainfall combined with moderate temperature and fertile soil support a luxuriant vegetation.
Located at (10°4'48"N 76°37'12"E) It is surrounded by the Archdiocese of Ernakulam and Madurai and the Dioceses of Kanjirapally, Palai, Idukki and Coimbatore. Situated in the centre of Kerala, the Eparchy of Kothamangalam with 4,800 square kilometers, the diocesan territory covers the taluks of Kothamangalam, Devikulam, Kunnathunad, Udumbanchola and Thodupuzha.
The diocese lies mainly on the highland consisting of the hills and forests respectively. The hilly or eastern portion is formed by a section of Western Ghats. Muvattupuzha and Periyar are the main rivers of which the latter flows through Thodupuzha, Muvattupuzha, Aluva, Kunnathunadu and Parur taluks. During rainy season these rivers are full and heavy floods affect the low-lying areas on the banks, but in the summer season they generally go dry and narrow. The Periyar is stretched over a length of 229 km.