You will frequently hear claims that the church unanimously rejected military service for Christians right until the time of Constantine. The reality is much more nuianced.
Many years ago, German theologian Adolph von Harnack wrote a short work called Militia Christi (Amazon affiliate link) in which he worked out what was actually said about military service. It was only within the last fifty years that this was translated into English.
The book is divided into two sections. The first looks at military imagery that is used for Christian faith and ministry. This is significant because if military service is a sin, it would be an odd image to use in a positive way. And yet it is often used to describe the work of Christians.
The second section looks at what the Church Fathers actually said about military service. The truth is that for the first one and half centuries we have no comments on the issue. We cannot say one way or another what the earliest Christians thought.
We eventually get some Fathers writing on military service, likely because it was becoming more common. Both Tertullian and Origen rejected the idea that Christians could serve in the military. Yet we see comments in other Fathers that suggest that Christians were serving in the Roman army.
There definitely were issues with it. But it is not always clear as to the problem with being in the military. There were specific problems with being in the Roman army, both because of the ties to Roman religion as well as some of the unethical things they were expected to do.
Were the early Christians all pacifists? Probably some were and some were not. Some began in the army and decided they must leave, sometimes paying a high price. Others tried to serve in such a way that was compatible with their faith.
I need to say that I do not always agree with Adolph von Harnack. There are things that he claimed in other works that I disagree with. He also had some German Nationalistic tendencies. But this book at least compiles the relevant texts and allows readers to make up their own minds.
I found Militia Christi to be a helpful resource in my own understanding of these issues.
