This is a stamp issued to commemorate St Thomas, one of the 12 apostles of Christ. The stamp was issued in December 1964, to mark the visit of Pope Paul VI to India to attend the 38th International Eucharistic Congress convened in Bombay (Mumbai). The Santhome Church, as the Basilica of the National Shrine of St Thomas is popularly known, is in Mylapore, Chennai.
Some information from the St Thomas Basilica website: "By Apostolic brief dated 16th March 1956, Pope Pius XII gloriously raised the Cathedral Church of the Archdiocese of Madras-Mylapore to the dignity and rank of Minor Basilica.
The present National Shrine of St.Thomasis built over the tomb of Saint Thomas the Apostle, who came to India in the year 52 AD. After preaching on the West Coast, he came to Chennai(formerly Madras), and suffered martyrdom on a hill at the outskirts of the city, today known as "St. Thomas Mount." His body was buried on the spot over which the present Basilica stands.
As for the church, that too has much history behind it. This lovely Gothic Church is an architectural treasure. Rising 155 feet from the ground, with a nave of 112 feet by 33 feet, and an imposing sanctuary 62 feet long and 33 feet wide, it is adorned with stained glass windows depicting St. Thomas and the other Apostles. Inside the sanctuary is a statue of St. Thomas seated."