In our modern times, we're used to the pervasiveness of big government. A government-issued ID an passport, perhaps with a few serial numbers and of course a picture, is enough to establish your identity to most relevant authorities, and entitles you to the rights and privileges of a citizen, resident, or whatever other role you play in society.

For the vast majority of human history, this was not the case. There was no such thing as a "Roman ID card", although there were cases where someone's identity could be proved with a document or other piece of writing. Let's assume you're talking about one of those cases.

Depending on the time period (the Roman Empire was around for a long time, after all), you might have been able to find a retired soldier, on his way to begin his life on the plot of farmland granted to him for 25 years of fine service. He would have possessed a diploma outlining his rank, areas of deployment and commanding officer. So far so good; steal that and you're golden, right?

Well, maybe, but your options are still pretty limited. The only real thing that you could do with it would be to proceed in his place to the farmland where he and his unit would be settled in their retirement, whereupon you could present your documents to the local magistrate and gain his farmland.

And shortly afterward, you'd be murdered, as army groups were almost always settled close together in their retirement, and your neighbors would have been soldiers that served with the poor guy you robbed and who would know you aren't him. Herein lies the crux of the question: up until very, very recently, most peoples' identity would be first and foremost proven by people who knew them personally. If you couldn't find someone to vouch for you, you were at best an outlaw, at worst a bandit.