The following information was taken from:
Nothing is known about St. Sebastian’s youth other than the fact he may have come from southern France and he was educated in Milan. He joined the Roman Army in 283 AD, ostensibly to be of service to other Christians who were being persecuted by the Romans. St. Sebastian distinguished himself and for his excellent service, he was promoted to serve in the Praetorian Guard to protect Emperor Diocletian.
While serving as a Praetorian Guard, Marcus and Marcellian, twin brothers, were imprisoned for refusing to make public sacrifices to the Roman gods. The brothers were deacons of the Christian Church. During their imprisonment, their parents visited them to implore them to renounce Christianity. However, St. Sebastian convinced both parents to convert to Christianity. St. Sebastian also converted several other prominent individuals, including the local prefect.
This led to his discovery and he was reported as a Christian to Emperor Diocletian in 286. The Emperor, who was already infamous for ordering the deaths of hundreds of Christians, scolded Sebastian and ordered him to be killed by having him tied to a stake on a training field and used as target practice.
Archers riddled his body with arrows, his body was described as, “full of arrows as an urchin.” Believed to be dead, the archers left his body for retrieval and burial. He was recovered by Irene of Rome, whose Christian husband was a servant to Diocletian and also martyred. Irene discovered that Sebastian was still living and she hid him and nursed him back to health.
Soon after, Sebastian intercepted the Emperor, denounced him for his cruelty to Christians, and was beaten to death on the Emperor’s orders.
Saint Sebastian was venerated at Milan as early as the time of St. Ambrose and was buried on the Appian Way. He is patron of archers, athletes, and soldiers, and is appealed to for protection against plagues.
[He is patron saint of athletes because of his physical endurance and his energetic way of spreading and defending the Faith. Sebastian is also patron to all soldiers. He entered the Roman army under Emperor Carinus in 283 in order to defend the confessors and martyrs of his day without drawing attention to himself. His efforts kept the Faith of Marcus and Marcellian firm during their persecutions, right up to the time of their martyrdom. He was declared patron of plague sufferers of his reported cures of those afflicted with many diseases.]