In a land area of 15222 square kilometers, the diocesan territory covers the civil districts of Karbi Anglong and North Cachar Hills Districts of Assam. Diphu is the biggest city in the diocese. Haflong is the other important town in the diocese.
In the diocesan area, the population is 1,001,390 in 2008. Karbi, Bodo, Kuki, Dimasa, Hmars, Garo, Rengma Naga, Tiwa, Man, Zeme, Naga, Biates, Hrangkhols, Kehlmas, Adibasi, Pnar, Khasi, Nepali, Bihari are the various ethnic groups in the diocese.
The various languages spoken in the diocesan area are Karbi, Hindi, Assamese, Sadiri, Garo, Tiwa, Bodo, Pnar, Kasi, Kuki, Dimasa and Nepali.
The Diocese of Diphu was erected on Dec. 5, 1983 detaching the District of Karbi Anglong from the Archdiocese of Shillong-Guwahati and the District of North Cachar Hills from the Diocese of Silchar. Father Mathai Kochuparampil SDB was appointed the first Bishop of this new Diocese. He was installed at Diphu on March 4, 1984. He passed away on March 4, 1992. Father Albano D'Mello was elected Diocesan Administrator on March 11, 1992. On June 24, 1994, Father John Thomas Kattrukudiyil was appointed the second Bishop of the Diocese. He was consecrated on Sept. 8, 1994. He was transferred to the newly formed Diocese of Itanagar in December 2005, however, he was appointed as the Apostolic Administrator of the Diocese of Diphu, as well. On Feb. 14, 2007, Father John Moolachira was appointed as the third bishop of Diphu Diocese. He was consecrated on April 15, 2007.
The oldest missionary presence in the Diocese is that of RNDM sisters (Congregation of the Our Lady of the Missions) at Haflong. The first group of sisters settled down in Haflong in 1911. They started a small boarding school, which later on was opened to the children of the British engineers and tea garden managers. But due to malaria and other fatal diseases prevalent in the region, the infant school was closed down. In the year 1918, in the month of March, St. Agnes School was re-opened by the sisters. A priest began to reside at Haflong and to serve as chaplain to the convent. According to a government letter, the school was meant for the Europeans and the Eurasians in the province and not for any general missionary purpose. Such restrictions remained till the independence of India.
The Salvatorian Fathers had adopted Khasi Hills as their main field of missionary activity. They reached the boundary of Karbi Anglong but did not really enter it. Mr. Langtuk Hanse from the village of Marjong, just a few kilometres from the present parish centre of Umswai, was the first to hear about the Gospel. On Jan. 25, 1914, he led a group of six people from Marjong to Umtyrkhang, in Khasi Hills and received baptism at the hands of Father Christopher Becker. These six in turn became apostles and on May 4, 1916, 31 others from the same village received the faith. On the 15th of the same month another 22 received baptism. In 1920, some of those who had received baptism came to settle in Umpanai (Amkachi); and thus a community was started there too. Later on a community came up also in Mynser. The work in this region was not restricted to the Karbis only. The Tiwas too responded to the faith. In 1950, the people of Bor Marjong and Umswai received the faith.
The first one to receive the faith in Block I (Jirikyndeng) was Mr. Joseph Millick and his four children. They went to the Raliang Mission and were baptized on March 6, 1942. They were the people of Umkyrmi. From here the faith spread to the Karbis of other villages.
In the Rongkhang area the first to receive the faith was John Kathar with a group of 27 others. Msgr. Marengo received this first group into the Church. In 1927, with the election of Mgr. Lepailleur CSC as the first Bishop of Chittagong, Cachar was made part of the new diocese. Bishop Lepailleur acquired a lease on a fairly large piece of land adjacent to the convent in 1930, and in the year 1943, he obtained another lease on a plot of land about 10 minutes walk from the convent which later became the headquarters of the Prefecture of Haflong and the present priests' residence.
The mission of Haflong was initially run by the Holy cross Fathers attached to the Canadian Province of the Congregation. In the year 1952, on Jan. 7, Haflong was raised to the status of a Prefecture Apostolic, with Mons. Gomes Breens as the first Prefect Apostolic. His area of jurisdiction consisted of Cachar Hills, the Mizo Hills and Tripura. This Haflong Prefecture Apostolic was raised to the status of a Diocese on June 26, 1968 with the Episcopal See at Silchar, and Most Rev. Denzil D'Souza as its first Bishop.
In July, 1974, Father Peter Bianchi SDB was appointed the parish priest of Haflong. With the erection of the Diocese of Diphu, the parish of Haflong, which comprised the whole district of North Cachar Hills, became part of it. The Christian community of this district of North Cachar Hills is made up of Karbi, Zeme Naga, Khasi, Adibasi, Mizo, Hmar, Dimasa and other tribal groups.
The first baptisms in Dokmoka region among the Karbis was at Dentaghat in 1969. In 1950 some of the Catholics came from Sojong to Diphu to settle here and thus a community was started here too. Later on, the Karbis at Japralangso, Rurlangso, Balipathar and other villages were contacted. The first baptisms in Chokihola region was in the year 1973.
Although the first baptisms were in 1914, the first parish was opened in Karbi Anglong in the year 1967 at Sojong. On Feb. 11, the parish of Sojong was inaugurated with Father John Mariae as the first parish priest. In 1971, the parish of Diphu was opened, in 1972 the parish of Dokmoka, in 1974 that of Chokihola and in 1977 the parish of Umswai. Japrajan was the last parish inaugurated before the erection of the Diocese of Diphu.
Today the Diocese has 19 parishes and 23 subcentres. The missionary team of the Diocese is made up of the Diocesan priests, the Salesians, the MSFS, the Jesuits, Dominicans, OCDs, the Franciscan Brothers, Religious women belonging to 18 different congregations, numerous full time catechists, village catechists and other youth leaders. The diocese is constantly endeavoring to move forward in the spirit of the first missionaries who planted the Gospel in this part of the world
The City is managed by Corporation. The villages and small towns are administered by elected local bodies called Panchayats and municipalities respectively.
The diocesan area is well connected in terms of transport infrastructure by roads and railways. The nearest airport is in Guwahati city.
Rupees 14,523 (as of November 2009 USD312) is the per capita income in the diocese's territory. Rice and Maize are also grown in the area. Major industries include Cement Production, Tea, Handloom and Textile, Livestock and Poultry.
Government and private operators provide extensive telecommunication facilities in the diocesan area. The diocese is well connected by local cable TV networks.
62.71 per cent is the literacy rate in the diocesan territory.