A poster on Slashdot claimed "Did you know that two thousand years ago a Roman citizen could walk across the face of the known world free of the fear of molestation? He could walk across the Earth unharmed, cloaked only in the protection of the words civis Romanus -- I am a Roman citizen. So great was the retribution of Rome, universally understood as certain, should any harm befall even one of its citizens."
This clearly seems like hyperbole, but is there any kind of interesting related truth?
This is hyperbolic, yes. But it is less so if you keep in mind the context that the majority of Romans considered Rome itself to be the extent of "the world".
The idea that a Roman could literally go anywhere on Earth without harm does not seem grounded in reality.
However, within the borders of Rome and in the surrounding mileage where the reach of Rome was infinitely possible, Roman citizens were afforded a wide berth of courtesy and consideration. The classic example of this is from the Bible, in Acts 22.
24 The tribune directed that he was to be brought into the barracks, and ordered him to be examined by flogging, to find out the reason for this outcry against him. 25 But when they had tied him up with thongs, Paul said to the centurion who was standing by, ‘Is it legal for you to flog a Roman citizen who is uncondemned?’ 26 When the centurion heard that, he went to the tribune and said to him, ‘What are you about to do? This man is a Roman citizen.’ 27 The tribune came and asked Paul, ‘Tell me, are you a Roman citizen?’ And he said, ‘Yes.’ 28 The tribune answered, ‘It cost me a large sum of money to get my citizenship.’ Paul said, ‘But I was born a citizen.’ 29 Immediately those who were about to examine him drew back from him; and the tribune also was afraid, for he realized that Paul was a Roman citizen and that he had bound him.
Here, all Paul had to do was claim he was a citizen and they immediately relented the flogging. They were under the impression he was not, as citizenship was reasonably rare among conquered populations, especially so far away from the heart of the empire.
The earlier and earlier you look at Rome, the more exclusive and privilege-rich the status of citizenship was. As she aged, Rome began to grant citizenship more freely as a rite of reward for service, for art, for philanthropy, for military success, for alliance, for payment, and so on.
The status of "civis Romanus" seems to have only worked where Rome could actually reach. Where they could meaningfully retaliate if a Citizen of Rome were threatened or killed without the proper courtesies.