St. Demetrius of Thessaloniki (Άγιος Δημήτριος της Θεσσαλονίκης in Greek) was a Christian martyr who lived in the 4th century. He is also referred to as "the Megalomartyr" (Great Martyr) by the Greeks.
Demetrius was a general in the Roman army and a high official in Thessaloniki. He was arrested for preaching the gospel and converting pagans even after an edict was issued prohibiting Christians from doing so. While imprisoned, a young Roman soldier named Nestor, who was a Christian disciple of his, visited him in his cell. Nestor asked Demetrius to bless him as he had made up his mind to duel against a pagan named Lyaeus, who was challenging any Christian to fight him. Demetrius blessed young Nestor with the words: "You will defeat Lyaeus and win a crown of martyrdom". Nestor then proceeded to challenge Lyaeus and with the words "God of Demetrius, help me!" attacked and killed his pagan opponent.
At this, emperor Maximian order the execution of both Nestor and Demetrius. Saint Nestor was beheaded while Saint Demetrius was run through with spears ca 306.
Demetrius was extremely popular in the Middle Ages and, along with Saint George, he was the patron of the Crusades.
St. Demetrius' relics are kept in Thessaloniki, Greece, a city of which he is the patron saint. His memory is celebrated on the October 26. He is also revered by the Serbian Orthodox Church as Mitar, having a feast of Mitrovdan on November 8th.
St. Demetrius is also commonly spelled as St. Dimitrios by Greek migrants in Australia and the United States, and commenly depicted in icons with the White Tower of Thessaloniki in the background. The tower depicted in the icons was build in the 16th century, centuries after his life, as the architecture of the older tower in the exact site is unknown.