As lord of the Omura domain in the province of Hizen in Kyushu, he had long been interested in the prospects of foreign trade, and in 1562 he opened the harbor of Yokoseura in Hizen for the use of Portuguese trading ships and built a church there.
In 1570 on the advice of Father Cosmo de Torres he opened the port of Nagasaki, and in 1580 donated the area around Nagasaki and Mogi to the Jesuits, so that the region became a center of both trade and missionary activities. In 1582, on the advice of the religious inspector Valignani, Sumitada joined with Otomo Sorin, Arima Harunobu, and others in sending a mission to the Pope in Rome to express reverence and report on the progress of the Church in Japan. The mission consisted of a fifteen-year-old boy named Ito Mancio, a fourteen-year-old named Chijiwa Miguel, and two other youths.
In the fifth month of 1587, however, both Omura Sumitada and Otomo Sorin died, and the same year Toyotomi Hideyoshi issued an order banning Christianity, initiating an era of persecution and martyrdom for the Christians in Japan.
In 1563 he was baptized and took the Christian name Bartolomeo, thus becoming the first of the Christian daimyo.