Troparion Tone 3
The world has found you to be a great defense against tribulation
and a vanquisher of heathens, O Passion-bearer.
As you bolstered the courage of Nestor,
who then humbled the arrogance of Lyaios in battle,
Holy Demetrius, entreat Christ God to grant us great mercy.
Kontakion Tone 2
God, who has given you invincible might,
has tinged the Church with streams of your blood, Demetrius!
He pre-serves your city from harm,
for you are its foundation!
From the OCA website:
The Great Martyr Demetrius the Myrrh-gusher of Thessalonica was the son of a Roman proconsul in Thessalonica. Three centuries had elapsed and Roman paganism, spiritually shattered and defeated by the multitude of martyrs and confessors of the Savior, intensified its persecutions. The parents of St Demetrius were secretly Christians, and he was baptized and raised in the Christian Faith in a secret church in his father’s home,
By the time Demetrius had reached maturity and his father had died, the emperor Galerius Maximian had ascended the throne (305). Maximian, confident in Demetrius’ education as well as his administrative and military abilities, appointed him to his father’s position as proconsul of the Thessalonica district. The main tasks of this young commander were to defend the city from barbarians and to eradicate Christianity. The emperor’s policy regarding Christians was expressed simply, “Put to death anyone who calls on the name of Christ.” The emperor did not suspect that by appointing Demetrius he had provided a way for him to lead many people to Christ.