What were the Church’s views on participating in military service after the reign of the Emperor Constantine?

Eusebius of Caesarea (c. 260–340 A.D.) considered Constantine to be the representative of God on earth. He says, “And as he who is the common Saviour of mankind, by his invisible and Divine power as the good shepherd, drives far away from his flock, like savage beasts, those apostate spirits which once flew through the airy tracts above this earth, and fastened on the souls of men; so this his friend [i.e., Constantine], graced by his heavenly favor with victory over all his foes, subdues and chastens the open adversaries of the truth in accordance with the usages of war.”⁹⁶⁷ He also tells us that Constantine would pray to the Lord Christ before entering the battle.⁹⁶⁸

Eusebius offers a solution to the problem of the two differing attitudes among Christians towards military service. A Christian can follow the path of the laity, who can partake of pure marriage, just war, and civil service. Or, a Christian can take the path of priesthood, which demands virginity, poverty, separation from the world, and complete devotion to God.⁹⁶⁹