Name change on enlistment

[Renatus]

It does seem that a lot of people in Egypt had two different names. It does seem odd though - why didn't he just call himself Julius Isidorus, which seems perfectly reasonable? Perhaps that would imply he'd gained citizenship... but calling himself Julius Martialis would make it appear that way anyway. Legal thing, maybe?

(was I right about the Concordia inscription, by the way?)

My guess would be that the recruit was given a Roman name by the authorities on enlistment as a means of acculturation into the military environment by seeking to suppress his pre-enlistment identity. Egypt is unusual in the amount of written evidence it produces, so it is not possible to say how far this applied elsewhere in the Empire. It seems that, nevertheless, some recruits maintained their earlier identities, especially when dealing with other members of the family. Another example is P. Cornell Inv. no. 1.64.1, in which Valerius Paulinus, formerly Ammonas, wrote to his brother using both names, even though he had completed 25 years' service. I don't think that giving a recruit a Roman name implies that he was also granted citizenship.

I think that you are probably right about the Concordia inscription. Although there are two names, the inscription is couched in the singular.