St. Demetrius is a Christian martyr and saint, celebrated primarily in the Eastern Orthodox Church. He is believed to have lived in the 4th century in Thessalonica, where he was known for his strong faith and dedication to spreading Christianity. His life ended when he was imprisoned and ultimately executed for refusing to renounce his beliefs. His feast day is observed on October 26, and he is often depicted in iconography as a young soldier, symbolizing his bravery and commitment to his faith. St. Demetrius is also the patron saint of Thessalonica and is invoked for protection against enemies and illness.