Citizens and Foreigners
Whether a person had Roman citizenship or not had far-reaching social and legal consequences. Many non-citizens therefore did what they could to attain citizenship, which was possible for those committed to achieving that status. Caracalla granted Roman citizenship to all free inhabitants of the empire when he issued the Constitutio Antoniniana edict, putting an end to discrimination for many people.
The information below is taken mainly from work published by Barbara Pferdehirt, listed here under Further Reading.
Social stratification in the Roman Empire
Inhabitants of the Roman Empire lived within a complex legal and social system with varying privileges. Affiliation with a certain class was generally inherited, but social mobility was nevertheless characteristic of Roman society during the early and middle imperial periods. An active commitment to the Roman state could enhance opportunities for social advancement.
A distinction was made between persons who were freeborn, those who had been released from bondage, and slaves. But there was also further differentiation within these groups. The freeborn were divided into Roman citizens and peregrini (foreigners). Some provinces were also home to an intermediary social class of people who had been granted a special legal status that had evolved historically and was known as Latin rights.
Becoming a Roman citizen
Children born to parents who were both Roman citizens were also Roman citizens by birth.
The emperor could, of course, and did grant Roman citizenship, especially when someone had served the Roman state in some outstanding way or might prove useful for political goals. Advancement could also take place over several generations, depending on the original circumstances, and monetary aspects also played a role.
Another path to citizenship was by completing at least twenty-five years of military service, assuming of course that the soldier survived.
Constitutio Antoniniana: Citizenship for (almost) everyone
The Constitutio Antoniniana granted citizenship in 212/213 to all free inhabitants of the Roman Empire. The exceptions were of course slaves and some people freed from bondage known as Latini luniani, and some foreigners who had been settled in the empire at a later time, for example, to serve in the military. Especially for the peregrini (foreigners), this marked the end of discrimination as far as their legal status was concerned.
Kuhlmann and Barnes (Kuhlmann and Barnes 2012, 50) have point out that Caracalla’s edict may have diminished the status of Roman citizenship rights. Moreover, not all social groups in the population benefited from the edict, which may have been reflected in opinions expressed by those speaking for such groups, for example, by the historian Cassius Dio.
The next section describes the relevant >>differences in legal status<< in more detail.