DEMETRIO of TESSALONICA, saint. - A martyr under Massimiano about 306, whose cult, introduced belatedly into Tessalonica by the prefect of Illyricum Leonzio at the beginning of the fifth century, immediately acquired the greatest renown throughout the Greek world, both as a military saint and as patron of that metropolis, which was believed to possess his relics. The copious literature that developed around the senatorial and consular status of the personage, as well as his cult, is founded solely upon a threefold series of later legends.
The early martyrologies knew of no St. Demetrio martyred at Tessalonica, but at Sirmio (Syrian Martyrology, 9 Apr.), without particulars or with the designation of deacon (Hieronymian Martyrology, 9 Apr.). Was D. therefore originally from Sirmio, where his cult was found earlier, and where Leonzio Illirico also caused a church to be erected? Were only some relics, and not the body, transferred to Tessalonica, while the latter remained at Sirmio, at the place of martyrdom? Why did the transition take place from a saintly deacon to a military saint? Such are the problems raised by the comparative study of the legendary accounts concerning St. D. in the documents now known. It is certain that the basilica erected by Leonzio in Tessalonica was throughout the Middle Ages a destination for pilgrimages. In the Roman Martyrology, his feast is on 8 October.