Throughout the history of Rome, citizenship was a desirable status to have. Wars were fought over this issue. Even as late as the early part of the first century it seems like it was an uncommon and privileged status in the provinces.
In 212 C.E. citizenship was extended to every free person in the entire empire.
The Edict of Caracalla spends some time talking about how divine and magnificent the Emperor is. I’ve skipped that and cut to the chase, but you can get the full text and more commentary here. But here’s the important parts:
Imperator Caesar Marcus Aurelius Seuerus Antoninus Augustus dicit: [. . .] Do igitur omnibus peregrinis, qui in orbe terrarum sunt, ciuitatem Romanorum.
Emperor Caesar Marcus Aurelius Severus Antoninus Augustus says: [. . .] I give, therefore, to all foreigners, who are in the sphere of lands, Roman citizenship.
dicit — This is the third person singular of “to say.” The site I linked uses the more legalistic “proclaims.” Note there’s no plurality as in the royal “we” you might find in Europe during later periods.
imperatum — Imperium existed before the Empire during the Republican period. It was the legal power to command troops vested in magistrates like the consuls.
Do — I give. Cf. “donation”
omnibus peregrinis — all foreigners. We use ‘omnibus’ in English unchanged. ‘Peregrinis’ generally comes from a meaning of ‘to wander about the fields”, from per + ager (field).
qui in orbe terrarum sunt — who are in the world. ‘Qui’ here is the same as the ‘qui’ we see in Spanish and French, but it’s pronounced ‘kwee’ not ‘kee.’ ‘Orbe terrarum’ means something like ‘sphere of lands.’ It basically means ‘the world.’ The word ‘orbe’ is used but it doesn’t signify a concept of a 3-dimensional spherical world, but rather a circular flat disc-shaped world. We get ‘terrestrial’ and other cognates from ‘terrarum.’
Romanorum — It may seem evident, but I want to point out the Romans described themselves as ‘Roman.’ It isn’t always the case that the word we use to describe an ancient society is the same word that society used itself. For example, Greek/Hellenic.