The Diocese of Ilagan was created on January 31, 1970 and canonically erected on May 12, 1970, comprising the civil province of Isabela. It is a suffragan of the Archdiocese of Tuguegarao. Its titular patron is St. Ferdinand whose feast the diocese celebrates on May 30. The land area covered by the diocese is 10,664 square kilometer, and its population is now 1,120,235 of which 76 per cent are Catholics.

On May 12, 1995, the Diocese of Ilagan celebrated the 25th anniversary of its canonical erection. Just before, in April, the first diocesan pastoral assembly was held. The implementation of the decrees of the Second Plenary Council of the Philippines and the renewal of the diocese in the faith were the twin pillars of the pastoral assembly.

There are 28 parishes and 8 quasi-parishes today in the diocese, served by 54 priests and 54 religious sisters working in related activities. Its Catholic institutions include 3 colleges, 11 kindergarten, 9 elementary and 16 high schools, and 7 pastoral centers. Basic Ecclesial Communities now cover 87 communities.

The history of Christianity in the province of Isabela is intimately linked with that of its sister provinces in the Cagayan Valley region, namely Cagayan and Nueva Vizcaya. All three provinces were just one political unit called Cagayan during the early Spanish period.

Mission stations in the area which later became settlements were founded by Augustinian and Dominican friars as early a 1717. An earlier attempt at evangelization had been made in a little place called Ituy, found in what is now Nueva Vizcaya. Through the efforts of the Augustinians and the Dominicans the faith spread over the region.

In 1841 the province of Nueva Vizcaya was created occupying the entire lower half of the Cagayan Valley. This divided the region into two provinces, the demarcation line cutting horizontally across what is now Isabela. Until 1856 Isabela was just part of the provinces of Cagayan and Nueva Vizcaya.

In 1856, Isabela Province was created, deriving its territory half from Cagayan and half from Nueva Vizcaya. Isabela is now the central province of the Cagayan Valley known as Region II. The entire region is traversed longitudinally by the Cagayan River, the largest and longest river in the country which the Spaniards called the Rio Grande , or large river. The capital of Isabela became Ilagan.

During the Philippine Revolution of 1896, Isabela was known to be a stronghold of Filipino forces under Colonel Daniel Tirona against Spain. But Isabela is more significantly remembered in Philippine history as the place where General Emilio Aguinaldo, the president of the First Philippine Republic, was captured by American forces in 1901, in the town of Palanan. His capture officially ended the existence of the First Philippine Republic.

Today Isabela is one of the rice granaries of the country. Until the imposition of the log ban by the Government to preserve and replenish Philippine forests, Isabela ranked first in log production. Its inhabitants are a mixture of Ibanags, Tagalogs, Pangasinenses, Pampangos, and Gaddangs.