Sebastian was the son of a wealthy Roman family. He was educated in Milan and became an officer of the imperial Roman army, and Captain of the Guard. He was a favorite of Emperor Diocletian. During Diocletian's persecution of the Christians, Sebastian visited them in prison, bringing both supplies and comfort. He is reported to have healed the wife of a fellow soldier by making the sign of the cross over her. During his time in the army he converted many soldiers and a governor.

Charged as a Christian in 288 in Rome, Sebastian was tied to a tree, shot with arrows, and left for dead. However, he survived, recovered, and returned to preach to Diocletian, where the emperor then had him beaten to death.