"O Lord, Who fought alongside the greatly-meek David..."

The Holy Cross of Christ (source)

  
O Lord, Who fought alongside the greatly-meek David, warring together with Your faithful people against the foreign king, through the weapon of Your Cross, depose our enemies, and show forth Your ancient mercies upon us, O Compassionate One, that they may truly know that You are God, and through hope in You we will be victorious, and through the intercessions of Your Spotless Mother, grant us the great mercy.
-Doxastikon from the Aposticha of Great Vespers for the Veneration of the Cross

(source)

Through the prayers of our Holy Fathers, Lord Jesus Christ our God, have mercy on us and save us! Amen!