In view of 2 Tim. 4:20, it is quite likely that Erastus was a Christian. Although Rom. 16:23 does not provide detailed information about Erastus, in context, it seems that Paul includes him as a member of the early congregation. However, the foregoing being said, we must not overlook what the Greek text and history possibly tells us about this man.
Erastus was possibly the city-treasurer in Rome; the words used to describe this man are ὁ οἰκονόμος τῆς πόλεως. There is a world of difference between being a treasurer and being a soldier, which is how some want to characterize Erastus. While the distinctions JUS AD BELLUM and JUS IN BELLO became popular with Augustine of Hippo and his formulation of just war theory, we have evidence that the early Christians had no such philosophy--they applied the words of Isa. 2:2-4 to themselves and put down their weapons of war.
For what its worth, William Mounce does quote an inscription in his commentary indicating that Erastus could have been an aedile. But could is the operative word in this case, for we are not sure if the inscription cited is talking about the same individual.