He was born the second son of Arima Harusumi, a warring-states daimyō of Hizen Province. The Arima were a powerful clan ruling the Shimabara Peninsula, and Arima Harunobu, who would also become a Kirishitan daimyō, was his nephew. The fact that his mother was a daughter of the Ōmura clan would later change Sumitada's fate greatly.
Through his mother's family, he was adopted as heir by his uncle Ōmura Sumisaki. Sumisaki had a son by a concubine, but that boy was sent for adoption to the Gotō clan of Takeo and took the name Gotō Takaakira. Denied the succession, Takaakira bitterly resented Sumitada and later became his long-standing enemy.
He succeeded his adoptive father as head of the Ōmura and made Sanjō Castle his base. The Ōmura were a small daimyō house in Hizen, surrounded by the powerful Ryūzōji and Matsura clans as well as opposing factions among their own retainers. Protecting this weak house was the great task facing the young Sumitada.
After an incident in Hirado in which Portuguese were killed and wounded, Portuguese ships were looking for a new port, and Sumitada offered Yokoseura in his own territory. Knowing that missionaries had great influence over the merchants, he also gave the Society of Jesus housing and other favors, and the port flourished.
After studying Christian teachings with the missionaries, he received baptism with his retainers from Father Torres, took the baptismal name Bartolomeu and became Japan's first Kirishitan daimyō. His conversion is thought to have been partly motivated by the profits of trade, but records describe Sumitada himself as a devout believer all his life.
His destruction of temples and shrines in his domain and his urging of Christianity upon his people provoked fierce opposition from Buddhist retainers. Gotō Takaakira joined with such retainers and rebelled, and the thriving port of Yokoseura was burned down. Sumitada's faith was extremely zealous, and his forceful methods created much conflict.
As a new port, he offered Nagasaki, ruled by his retainer and son-in-law Nagasaki Sumikage, to the Portuguese and the missionaries. The next year a Portuguese ship entered the port for the first time, and Nagasaki, until then only a lonely fishing village, grew into a great port of the Nanban trade. This is considered the beginning of today's city of Nagasaki.
With reinforcements from the Matsura and others, Gotō Takaakira made a surprise attack on Sanjō Castle, where Sumitada was. It is said that only about eighty of Sumitada's men were in the castle against fifteen hundred enemies, yet they held out until help arrived and drove the enemy off. This battle is remembered as the "seven horsemen holding Sanjō."
Under pressure from the expanding Ryūzōji Takanobu, Sumitada had been forced into near submission, even sending his sons as hostages. This year, when the port of Nagasaki was attacked by Ryūzōji and other forces, Sumitada drove them off with Portuguese help and kept the trading port safe.
Fearing that neighboring daimyō would seize the port, Sumitada donated Nagasaki and Mogi to the Society of Jesus as church territory. It is thought to have been a decision to protect the port's safety and the profits of trade. This donation later heightened Toyotomi Hideyoshi's suspicion, and Nagasaki was made Hideyoshi's direct territory.
He met Valignano, the Jesuit visitor touring Japan, and with Ōtomo Sōrin and Arima Harunobu decided to send the Tenshō embassy of boys to Rome. His nephew Chijiwa Miguel was chosen as Sumitada's envoy. After a long voyage the boys would meet the Pope in Rome.
In the Battle of Okitanawate between Ryūzōji Takanobu and the allied forces of Arima Harunobu and the Shimazu, Sumitada had no choice but to send troops on the Ryūzōji side. But he avoided truly fighting his Arima relatives, and his men are said to have fired guns without bullets. Takanobu lost his life on the battlefield, and Sumitada was freed from Ryūzōji pressure.
In Toyotomi Hideyoshi's pacification of Kyushu he submitted to Hideyoshi and kept his lands, but Sumitada was already gravely ill, and his son Yoshiaki went to war in his place. Sensing death, he is said to have freed his prisoners and had a beloved little bird released into the sky. He died at his residence in Sakaguchi the month before Hideyoshi issued the edict expelling missionaries.
Ryuzoji TakanobuŌtomo SōrinToyotomi Hideyoshi
Luís FróisKobayakawa TakakageAsakura YoshikageShimazu YoshihisaOda NobunagaHosokawa Yūsai (Fujitaka)Shimazu YoshihiroNiwa Nagahide
People of the Sengoku/Azuchi-Momoyama period ・ People by period
Based on facts found in textbooks and biographies. Uncertain points are marked as such.