Summary:

The commandment of the Law was to love your neighbor (Leviticus 19:18), but the Lord commanded to love your enemy. Christians can not slay their enemies, or condemn them to be burned or stoned. The Lord took from the Jews the right of making war upon their enemies, of fighting for their country, of putting to death or otherwise punishing adulterers, murderers, or others who were guilty of similar crimes (ORIGEN). 

The early Christians such as JUSTIN MARTYR, IRENAEUS, TERTULLIAN, and ORIGEN understood the New Covenant commandment to “love your enemies” to be a fulfillment of the prophecies, “neither shall they learn war any more,” in Isaiah 2:4 and Micah 4:3. 

Based upon this command to “love your enemies,” the early Christians were without exception opposed to participation in warfare. 

Christians refrain from making war upon our enemies (JUSTIN MARTYR). 

It is not in war, but in peace, that Christians are trained (CLEMENT OF ALEXANDRIA). 

Soldiers of the civil authority must be taught not to kill men and to refuse to do so if he is commanded (HIPPOLYTUS). 

After the name of Christ was heard in the world, wars diminished (ARNOBIUS). 

Unlike those who strive to be serviceable to their country, a just man does not injure, hate, despoil, or put to death (LACTANTIUS). 

Yet the early Christians also mention the existence of Christian soldiers1 The quotations from TERTULLIAN (3.100) and HIPPOLYTUS (Apostolic Tradition, 16) explain the seeming contradiction. As Christians evangelized the Roman Empire, many pagan Roman soldiers were converted. Once baptized, those who were already in the military could not simply resign because leaving the Roman army was punishable by death. Although joining the military was disallowed by the early Church, they permitted converted soldiers to remain in the army after baptism so long as they did not use the sword, swear oaths, or participate in idolatrous practices.

Even the Nicene Council forbid military service and set a penance for those who violated their former repentance. The First Council of Nicaea (325) declared,

As many as were called by grace, and displayed the first zeal, having cast aside their military girdles, but afterwards returned, like dogs, to their own vomit, (so that some spent money and by means of gifts regained their military stations); let these, after they have passed the space of three years as hearers, be for ten years prostrators. But in all these cases it is necessary to examine well into their purpose and what their repentance appears to be like. For as many as give evidence of their conversions by deeds, and not pretense, with fear, and tears, and perseverance, and good works, when they have fulfilled their appointed time as hearers, may properly communicate in prayers; and after that the bishop may determine yet more favorably concerning them. But those who take [the matter] with indifference, and who think the form of [not] entering the Church is sufficient for their conversion, must fulfill the whole time.

Though people's commitment to Christianity changed dramatically during the reign of Emperor Constantine, the Council of Nicaea is nonetheless representative of the dominant orthodox views of the broader church at the time. If they were disciplining disciples who failed to maintain their repentance necessary to enter the church, then how can an argument possibly be made that the post-apostolic church had no problem with military service?

Jesus commanded His disciples not only not to hate men, but also to love their enemies (IRENAEUS). 

Christians love all men, and are persecuted by all (MATHETES). 

Loving one’s enemies does not mean loving wickedness, or impiety, or adultery, or theft; but the thief, the impious, and the adulterer 

(CLEMENT OF ALEXANDRIA). 

Pray for those that persecute and hate you, and for the enemies of the cross, that your fruit may be manifest to all, and that you may be perfect in Him (POLYCARP). 

If we are enjoined, then, to love our enemies, whom have we to hate? (TERTULLIAN). 

The Lord proclaims the Father to be good (CLEMENT OF ALEXANDRIA, ORIGEN, TERTULLIAN). Disciples are called to imitate God as His children (MATHETES, ORIGEN, CYPRIAN). 

The Lord was giving precepts for salvation and instructing His disciples to perfection (CYPRIAN). 

The Father wishes us to be perfect by aiming at sinlessness and living blamelessly, according to the obedience of the Gospel

(CLEMENT OF ALEXANDRIA).