Demetrius (d. 306) was a Christian saint, a native of the city of Thessalonica in Greece, where he was martyred while Greece, like other parts of the Roman Empire during the later years of Diocletian's reign, experienced persecutions against Christians. According to tradition, he was run through by spears. Later traditions, which may or may not have a factual basis, made him a Roman soldier, and he became a warrior saint associated with Saint George.
Veneration for him became very widespread in the Middle Ages.
St. Demetrius in Thessalonica
Demetrius became the patron saint of his home city, Thessalonica in the centuries after his death. His presence was clearly felt in the church dedicated for him. Members of the city's civic militia, on whom defense of the city fell since the garrison of regular soldiers was removed, felt a stronger confidence and martial spirit after visiting the church.
When the Slavs and Avars sought to take the city by surprise, the saint took over the body of the militia leader Rufus, alarming the citizens and getting them to man the walls in time. Later, when the Slavs attempted to breach the wall with their siege engines, Saint Demetrius endowed the seventy-year old Rufus with the strength to lift heavy rocks - which even much younger men normally could not do - and drop them on the attackers. Also other militia members sometimes got the aid of the Saint in shooting their arrows more far and more accurately than they could have by themselves.
When the attackers made their ultimate move, calling upon the Slavic gods to manifest themselves and destroy Thessalonica, Demetrius manifested himself to battle the three-headed Slavic god Triglav. The Saint rode out of the city, wearing an archaic Roman armor, and engaged Triglav in battle. However, before the battle could be decided the Centaurs - driven to fury by drinking wine, for the first time in many centuries - arrived and trampled the Avar and Slav sorcerers who had called up the Slavic gods. With their death, Triglav and the other Slavic gods disappeared and so did St. Demetrius.
Aside from his warrior aspects, St. Demetrius was also a healing Saint, controlling a spring outside Thessalonica which prior to the advent of Christianity belonged to the healing god Asclepius. Shortly before Thessalonica came under siege, St. Demetrius performed a miracle there and restored the long-paralyzed legs of the noble Menas. George the shoemaker, among other Thessalonicans, wondered why St. Demetrius (and God) singled out the mean-spirited Menes for such a miraculous cure. Long afterwards, George speculated that it might have been part of a very convoluted divine plan to save Thessalonica from the siege.
When meeting with the Centaurs, George was surprised to find that a centaur-colt was named "Demetrius". Wondering if that meant the centaurs were trying to be reconciled with Christianity, he asked if the colt was named for the Saint - but was rebuked and told the name was in reverence for Demeter, a Great Mother Goddess of the ancient Greek Pantheon. Of course, the Saint's name had ultimately been derived from the same origin, though Christians did not like to refer to this fact.