The church is dedicated to St. Sebastian, a Roman martyr. There is only one other such dedication in England and this is at Wokingham.
The story of St Sebastian is that of a brave double agent - a man who, although a Christian, kept his views secret during the persecutions of the Emperor Diocletian, not out of fear but in order to be able to comfort those who were daily being martyred for their faith. Born in Narbonne, France, he enlisted as a Roman soldier in the year 283, even though some of his duties seemed to cut across his Christian faith.
Secretly, he encouraged Christians about to be killed not to waver in their beliefs. Even more boldly, he converted such noted pagans as a man named Nicostratus, who was in charge of the prisoners, and his wife, Zoe.
Sebastian cured the prefect of Rome of his painful gout, and the prefect himself became a Christian, setting many godly prisoners free. Yet still Sebastian remained undetected. The Emperor Diocletian heard favourable reports of this young soldier and ordered that he be made a captain in the Pretorian Guards.
This could not last. First, Zoe was discovered to be a Christian. Then Nicostratus and the converted prefect were captured and killed. Finally the faith of Sebastian was discovered. Diocletian is said to have been bitterly disappointed that one so close to him had been a believer in Christ. The saint was sentenced to be shot to death by arrows. In spite of his many wounds, he survived to be nursed back to health by a widow named Irene.
But Sebastian now was determined to confront the great persecutor of his fellow Christians. He publicly appeared before a startled Diocletian to attack the Emperor’s cruelty to Christians. For a moment Diocletian could hardly speak. Then he sentenced Sebastian to be clubbed to death. The saint’s body was cast into a sewer, but Christians rescued it, burying it on the Appian Way at the spot where the church of San Sebastiano stands today.
People believed that St. Sebastian could protect them from disease and pestilence and about the time of the dedication of the church, the plague, (1665), was probably causing problems in Grantham and the surrounding hamlets, hence the unusual dedication.