12th-century mosaic depicting St Demetrios, from the Golden-Roofed Monastery in Kiev.
St. Demetrius of Salonica was a Christianmartyr who lived in the 4th century. He is also referred to as “the Megalomartyr” by the Greeks.
Demetrius was a deacon in Salonica. Early legends about Demetrius credit him with a military career, so he's also sometimes called a military martyr. He was run through with spears ca 306 at Sirmium, modern Serbia, during the Christian persecutions of emperor Maximian.
Demetrius encouraged Origen when blamed for his too literal execution of an allegorical counsel of our Lord, and is said to have shown him great favour.
Demetrius wrote to him complaining that this was unheard of presumption in a layman.
But Origen was ordained priest at Caesarea without leave, and Demetrius with a synod of some bishops and a few priests condemned him to banishment, then from another synod sent a formal condemnation of him to all the churches.