In a 1992 survey of recent work on the early church’s views on Christian participation in the military (in Religious Studies Review ), David G. Hunter sums up with this:

“the ‘new consensus’ would maintain: 1) that the most vocal opponents of military service in the early church (e.g., Tertullian and Origen) based their objections on a variety of factors, which included an abhorence of Roman army religion as well as an aversion to the shedding of blood; 2) that at least from the end of the second century there is evidence of a divergence in Christian opinion and practice and that Christian support for military service (first reflected obversely in the polemics of Tertullian) grew throughout the third century; 3) that the efforts of Christians to justify participation in warfare for a “just” cause (most notably that of Augustine) stand in fundamental continuity with at least one strand of pre-Constantinian tradition.”