There are a substantial number of passages written over a period of many years that explicitly say that Christians must not and/or do not kill or join the military. Nine different Christian writers in 16 different treatises explicitly say that killing is wrong. Four writers in 5 treatises clearly argue that Christians do not and should not join the military. In addition, four writers in eight different works strongly imply that Christians should not join the military. At least eight times, five different authors apply the messianic prophecy about swords being beaten into ploughshares (Isa. 2:4) to Christ and his teaching. Ten different authors in at least 28 different places cite or allude to Jesus' teaching to love enemies, and, in at least nine of these places, they connect that teaching to some statement about Christians being peaceful, ignorant of war, opposed to attacking others, and so forth. All of this represents a considerable consensus.
Indeed, there is very little basis in the texts for describing the early Christian view as "divided and ambiguous." There are no authors who argue that killing or joining the military is permissible for Christians. On these questions, every writer who mentions the subject takes essentially the same position. Some pre-Constantinian Christian writers say more about these topics than others. Some do not discuss them at all. But to conclude from this relative silence or paucity of some surviving texts that other writers disagreed with the extant texts would be sheer speculation. The texts we have do not reflect any substantial disagreement. Every extant Christian statement on killing and war up until the time of Constantine says Christians must not kill, even in war.
That a growing number of Christians, especially in the late 3rd and early 4th centuries, acted contrary to that teaching is also clear. That in doing so they were following other Christian teachers and leaders who justified their conduct, we cannot deny with absolute certainty. But we have no evidence to support the suggestion that such teachers ever existed until the time of Constantine.
Furthermore, we have the clear, striking case of Lactantius, who clearly changed his mind after he joined Constantine. His Divine Institutes (started about 304 in the midst of Diocletian's persecution and while Lactantius served as a prominent professor of Latin rhetoric in Nicomedia, where the emperor lived) vigorously condemn every kind of killing (including capital punishment) and reject Christian participation in the army. But by 310, Lactantius has joined Constantine and is tutoring Constantine's son. In On the Death of the Persecutors (c. 313-315), Lactantius celebrates Constantine's military victories. And in later works, Lactantius omits any condemnation of warfare and defends rather than condemns capital punishment.
The case of Lactantius shows not that since Christians embraced and defended Constantine's military victories, they must therefore have not previously thought that killing in war was wrong. Rather, the story of Lactantius demonstrates that one of the most vigorous, uncompromising Christian teachers rejecting all killing in his writing before the time of Constantine could quickly change his views about Christians and killing after experiencing the astonishing end of persecution and embrace of Christianity by Constantine. [emphasis added] The case of Lactantius challenges the argument that since Christians quickly celebrated Constantine's military victories, there must not have been much earlier widespread Christian teaching against Christians being soldiers
It's a bit cynical, perhaps, but it's never amazing how a person's views can change when a patronage shift happens or something comes up. A guy who was anti-immigration for years might suddenly favor immigration reform if his wife is from overseas, for instance. Someone who thought a war was a great idea until his/her kid gets in a tour is another example.