Tough to say. The problem of the mid 4th century is that we don't know to what extent people were in a transitory Christian/"pagan" religious framework (Peter Brown's recent book "Through the Eye of a Needle," although ostensibly about wealth, gives an excellent summary to the problematic evidence of the mid 4th century, regarding Christianity). As in, they might've believed in Jesus, but they might have also had some leeway for traditional Roman deities. They were certainly still celebrating traditional religious festivals (this being a major complaint of 4th century bishops). The hard orthodoxy comes in the late 4th century, probably around Theodosius I, which although it's only a few decades later, is more politically fraught, and thus religious divides get mapped onto political ones.

But from a nominal perspective, a substantial number, perhaps even a majority of the army were considered "Christian" in the sense that they followed Constantine's patronage of a supreme Judeo-Christian deity and his divine/human son, Jesus. Just don't press those soldiers too deeply about exactly what it is they believe in.