The area under the Diocese of Kottar has known the Christian Faith several centuries before the arrival of the Portuguese on the West Coast. Monuments recently excavated in a place called China-Mutton amply testify to this fact. The presence of St. Thomas Christians in places like Thiruvithancode (presently a small sub-station in the diocese) and Kottar has been attested by several fathers under the Padroado.

However with the coming of Portuguese fathers a small section of the local people, whose descendants still form a significant part of the diocese, became Christians. They are known as Paravas. About 7 years after their conversion in December 1544, over ten thousand fishermen, known as Mukkuvas, living on the sea coast from Poovar to Pallam were baptised by one of the greatest fathers of modern times, St. Francis Xavier.

With the renewed efforts of Sacred Congregation of Propagation of Faith (founded in 1622) Christian fathers began to penetrate into the interior parts of the area. Initially attention was given to the conversion of the Nairs. There were several Christians those days from the Nair community especially around Nemon, five miles south of Trivandrum. It is to this community of early Nair converts that the great Martyr of the diocese, Devasahayam belongs.

The 18th century saw rapid growth of Christianity in the interior parts of the area, following a large-scale conversion of people belonging to Nadar community. Vencode and Karengadu were centres of such conversions. With the creation of the Vicariate of Verapoly the Church of Kottar came under the efficient supervision of the Carmelites. A Carmelite Bishop of Quilon, Bp Aloysius Maria Benziger, took pains to organise and strengthen the present Church of Kottar. Thanks to his recommendation to Rome, the present Diocese of Kottar was created in 1930 with Bp Lawrence Pereira as its First Bishop. He was the third native Indian in modern times to become a bishop.

Linguistically Tamil has always been the language of the people of the area. Keeping, therefore, the linguistic aspiration of the people, Rome sent, as successor to Bp Pereira, a Jesuit from Tiruchirapally, Most Rev. Thomas Roch Agniswamy. Evanglisation activities received impetus during his tenure. A great number of parishes, Church work stations and schools came into being. His special interest in the promotion of vocations ensured that by the time he left the diocese in 1971 practically all the priests of the diocese were from within the area.

Most Rev Marianus Arokiasamy who succeeded him was committed to the implementation of the reforms of Second Vatican Council especially in the fields of liturgy, catechetics, Bible and pastoral activity. The establishment of sixteen diocesan commissions, Pastoral centre, animation centre and youth centre and the erection of parish councils in more than fifty percent of parishes bear witenss to those interest. Conscious of the needs of the wider Church he gladly sent his priests to serve in various capacities at national and international level. He was promoted as Archbishop of Madurai in 1987.

Most Rev Leon A. Tharmaraj became the fourth bishop of the Diocese.

The diocese consists of the taluks of Thovalai, Agasteeswaram (minus the parish of Azhagappapuram belonging to the diocese of Tuticorin), Kalkulam and vilavancode (minus seven villages along the coast belonging to the diocese of Trivandrum) of the Kanyakumari District of Tamilnadu.