It was Bishop Francisco Alban, OP who founded the Seminario de San Pablo, later to become Immaculate Conception School of Theology (ICST), in 1822. The Bishop saw the serious lack of co-workers, in the missionary endeavor of the Diocese of Nueva Segovia, which was then made up of the whole northernmost frontier of the archipelago. Thus, he started the construction of a seminary located at the south side fronting the Cathedral of Vigan to provide training of young men for presbyteral service in Northern Luzon. In 1867, the seminary was placed under the patronage of the Imrmaculate Conception.

Since its foundation, the seminary has been served by a good number of priests and brothers of different Religious Orders and Congregations. First to serve as formators were the Dominicans who stayed from 1840 to 1872; next came the Vincentians who served from 1872 to 1876 The Augustinians came in 1876 to serve until 1898. In 1896 the seminario, was closed down because of the onslaught of the Philippine Revolution. It was opened on December 17, 1898 but barely a year later it was again closed down because of the coining of the American Forces who used the seminary as formators fell nexr to the Fathers of the Society of the Divine Word. The SVDs served a, formarors from 1925 to 1987. During three years, under the. SVDs the seminary achieved numerous and considerable progress. The whole seminary compound housed the three departments: Minor, Philosophy, and Theology. Old folks in Vigan still remember with nostalgia, the old seminario with its Veladas, the famous Orchestra, the Schola Cantorum and the once-a-week paseos.

A few years later after the war, the construction of a new seminary building was started. A sprawling four hectare lor in Pantay Daya, Vigan was chosen as the site of the new seminary. In 1952 the Philosophy and Theology departments were transferred from the old seminary to the new one. However, it was only in October 7, 1953 that it was inaugurated and was given the new name Immaculate Conception Major Seminary. The old seminary continued to house the minor seminarians until 1965. On January 7, 1968, the old venerable seminary was gutted down by fire after 146 years of serving the needs of the Northern Luzon Church.

In 1976, the Philosophy department was transferred to Baguio City and took the name San Pablo Seminary, the original name given by Bishop Alban.

In 1982, the seminary was renamed the Immaculate Conception School of Theology while serving at the same time as a Regional Seminary for Northern Luzon.

In 1987, the SVD Fathers gave the seminar) to the care of the Diocese Clergy. The SVD’s who stayed for 62 years is the longest tenure of all the religious congregations that served the ICST.

The ICST is affiliated with the Faculty of Sacred Theology of the University of Santo Tomas. Hence, she is granted the right to confer the ecclesiastical degree of Bachelor in Sacred Theology (STB) to deserving students. They must however maintain the scholastic average and other requirements in order to qualify for the ecclesiastical degree.