In orbe Romano qui sunt, ex constitutione imperatoris Antonini cives Romani effecti sunt.This means that citizenship was universal -- at least to free people. Roman citizenship had earlier been a prized possession which conferred privileges. The publicly avowed purpose of the edict of Caracalla was as a thanksgiving for Caracalla's escape from the alleged plot of his brother Geta. Dio Cassius, also contemporary, was skeptical of the emperor's motives. He believed Caracalla was seeking a way to expand his tax base. Controversies about the edict have emerged over time. Although Dio Cassius and Ulpian both say the citizenship grant was universal, there are documents that challenge this. The edict can also be viewed as a stage in the persecution of Christians since a side effect of Caracalla's edict was that Christians, previously free from laws against Romans practicing Christianity, were now subject to them.
Those who are in the Roman world, are made Roman citizens by the consititution of Emperor Antonius.
"The Dediticii of the Constitutio Antoniniana," by Herbert W. Benario. Transactions and Proceedings of the American Philological Association, Vol. 85. (1954), pp. 188-196.
"The Constitutio Antoniniana and the Persecutions under Caracalla," by Paul Keresztes. The American Journal of Philology, Vol. 91, No. 4. (Oct., 1970), pp. 446-459.