Saint Demetrios of Thessalonica. Ionian Islands School icon, 18th century. The origins of St. Demetrios are more obscure even than those of his ‘companion’ St. George, with whom he is frequently paired in art. Modern research supposes that he was probably a deacon martyred at Sirmium (circa 3rd century) and that he was later transformed by popular imagination into the great warrior saint, second only to St. George in popularity. His cult developed in Salonika.