From the end of the New Testament period to the decade A.D. 170-80 there is no evidence whatever of Christians in the army. The subject of military service obviously was not at that time controverted. The reason may have been either that participation was assumed or that abstention was taken for granted. The latter is more probable. The expansion of Christianity had taken place chiefly among civilians in the urban centers.... Add to these consideration the rigorism of the Church which throughout the second century would not readmit to communion penitents guilty of apostasy, adultery, or bloodshed, and the likelihood appears greater that the Church withheld its members from military service than that they were permitted to serve without a single reproach or penalty. (Roland Bainton, Christian Attitudes Toward War and Peace 67-8)

After AD180 there is a steady increase in the number of Christians in the Roman armies. Concurrently, as we have seen, the church authorities and theologians began to denounce Christian participation in warfare. In the words of Bainton, “all of the outstanding writers of the East and the West repudiated participation in warfare for Christians” (73). Some see an apparent contradiction in the fact that while on the one hand, the Christian writers oppose Christian participation in warfare, on the other hand, they speak matter-of-factly of the existence of Christians in the military. The quotation from Tertullian below [3.100] explains this ostensible contradiction. “Although disallowing Christians to join the military, the church (at least by the close of the second century) permitted converted soldiers to remain in the army after baptism--so long as they did not use the sword, take oaths, or engage in idolatrous practices.” (David Bercot, A Dictionary of Early Christian Beliefs 681)

Of course, if faith comes later and finds someone already occupied with military service, their case is different. For example, there is the instance of those whom John [the Baptist] received for baptism, and of those most faithful centurions. I mean the centurion whom Christ approved, and the centurion whom Peter instructed. Yet, at the same time, when a man has become a believer and faith has been sealed, there must be either an immediate abandonment of the military office, which has been the course of many--or else all sorts of quibbling will have to be resorted to in order to avoid offending God. And such quibbling is not allowed even outside of military service. (Tertullian 3.100)

But for a man, bare feet are quite in keeping--except when he is on military service. (Clement of Alexandria 2.267)

We, on the contrary, bring before you an emperor who was their protector. You will see this by examining the letters of Marcus Aurelius, the most serious of emperors. For, in his letters, he bears witness that the Germanic drought was removed by the rains obtained through the prayers of the Christians, who happened to be under his command. (Tertullian 3.22)

We sail with you, serve in the military with you, and cultivate the ground with you. (Tertullian 3.49)

The soldiers, crowned with laurels, were approaching. However, one of them was more a soldier of God. In fact, he was more steadfast than the rest of his brethren, who had imagined that they could serve two masters. His head alone was uncovered, for he held the useless wreath in his hand. By that peculiarity alone, he was recognized by everyone as being a Christian. (Tertullian 3.93)

Only understand that [the martyrs] include men and women, both young men and old, both maidens and elderly matrons, both soldiers and private citizens. (Dionysius of Alexandria 6.96)

Many were martyred for refusing to fight, as in the example of Maximilian. Others who were not facing war were martyred for refusing to burn incense to idols, and others simply for being Christians. However, most of the time Christians could navigate life in the military without having to commit idolatry. As we will see below, some emperors began to require idol worship in order to weed out Christians.

There was also a body of soldiers.... Along with them, there was an old man, Theophilus. These men had taken up their position in a group in front of the tribunal. A certain person was on trial as a Christian, and he was about to deny [Christ]. As a result, these men stood round about and ground their teeth and made signs with their faces and stretched out their hands.... Eventually, they quickly ran up to the bench of judgment and declared themselves to be Christians too. (Dionysius of Alexandria 6.100)

Then Diocletian, in furious passion, ordered not only all who were assisting at the holy ceremonies, but also all who resided within the palace, to sacrifice. And, if they refused, they were to be whipped. And further, by letters to the commanding officers, he ordered that all soldiers should be forced to the same impiety, under pain of being dismissed from the service. (Lactantius, 7.304, 305)

Diocletian said that it would be enough for him to exclude persons of that religion [i.e., Christians] from the court and the army. (Lactantius 7.305)

In the two quotations above, it is clear that ordinarily Christians in the military could avoid idolatry, but Diocletian used idolatry to discover and dispense with Christians. As the Roman historian Cadoux has pointed out, the number of Christians in the Roman military must have been relatively small, for no military commander would reduce his army by any significant number over a religious matter, particularly not a Roman military commander.

Let a bishop, a presbyter, or deacon who goes to the army and desires to retain both the Roman government and the priestly administration be deprived. For "the things of Caesar belong to Caesar and the things of God to God." (Apostolic Constitutions 7.505)

This was the rule in AD390, well after the beginning of the Constantinian age. It is interesting to note that even though the rule has changed allowing lay Christians to soldier, the Christian aversion toward bloodshed itself had not yet altogether disappeared. Within another 40-50 years, some one hundred years after the ascension of Constantine, all soldiers in the Roman army were required to be professing, baptized Christians. (Interestingly, these soldiers were baptized with their sword arm raised above the water; a persisting—though ineffectual—statement about the unchristian nature of warfare.)