A Roman was brought up to believe that being a Roman citizen was to have won first prize in the lottery of life, and that Romans had absolutely nothing to learn from barbarian [i.e. other] cultures. Getting past that mindset would be the biggest barrier.
As mentioned above, aggressive war, and violence generally, were acceptable means of solving problems, slaves were indispensable for most civilizations to function, women were firmly second-class beings, the right of the elite, i.e. equestrian and senatorial classes, to govern was taken as a given. Where you had come from and the nobility of your lineage was more important than your innate ability. Democratic politics did bear some resemblance to our own, the plebeians could and did vote, but only to endorse or reject the proposals of the Great Ones. Homosexuality they wouldn’t have had much trouble with, providing it was not too overt. Equal rights generally, would be much more difficult to accept and would probably be dismissed as a sign of a degenerate, declining culture.
There was a slight influx of people.
Other Empires he may understand, but other Empires that were part of Roman territory? Where did these traitors or barbarians get the gall to rebel against the Eternal City?
As Mk II points out, the structure of society may initially confuse him. Although looking a bit deeper, you can see ‘patrician’ and ‘plebeian’ classes.
Worth noting that this fellow is a soldier. I’m not sure he’d be all that consoled by the existence of Italy (I mean no offence to any Italians); The Empire’s heartland being reduced to a bunch of squabbling city-states like those decadent Greeks (Greek culture was held in high regard by the patrician classes, but as a soldier our hypothetical time traveller would have little time for their values)?! Half of Italy once controlled by some bizarre new equivalent of the Pontifex Maximus? Only reunified around 140 years ago? Kings?!
He’d probably be greatly saddened that the Empire, which in his time looked like this, had been reduced to a watered down shell of itself.
The Romans had some amazing engineering, particulary for the ancient world, so I don’t think grasping new technology would be a big deal for him.
And from what I understand, Romans weren’t as Dogmatic as their polytheism as most monotheists are about their relgions. It wasn’t so much as “Pray to the wrong god and you go to hades” as “Fail to appease the right god for the situation and s/he may smite you(or just not help you)”.
Why would they need to? There are more than a billion polytheists around today.
The first post I read on the subject that gets to the real problem. A Roman legionnaire would have a real problem not reflexively smiting the first bureaucrat who thwarts him over something or other. Once he got past that and other common expectations, the rest of it might look like magic, but can learn to use computers as well as my 80 year old aunt.
The most recent Science Times had an article about an anthropologist who studies a hunter/gatherer tribe in Africa. She was interested in what they do when the hunting and gathering gets bad. The answer is that they prepare gifts and go visit relatives 200 miles away who may have better pickings. On one recent occasion they wanted something (I think it was shoes) and they found somehow an operating cell phone and somehow managed to call someone in California whose makers they held and arrange the anthropologist (who was in Utah) would buy them and charge them to the Californian and she would bring them with on her next trip to Africa. Is there any reason to believe that the Roman would be less enterprising?