Paul was born a Roman citizen:
And the chief captain answered, With a great sum obtained I this
freedom. And Paul said, But I was free born (Acts 22:28).
This means that a) Paul's father was a citizen and or b) Tarsus was a free city in which every free man born there was a citizen. The commentators are divided on which possibility is preferred:
For example from Ellicott:
This implies that St. Paul’s father or grandfather had received the
citizenship; how, we cannot tell. Many of the Jews who were taken to
Rome by Pompeius as slaves first obtained their freedom and became
libertini, and afterwards were admitted on the register as
citizens...The mention of kinsmen or friends at Rome...makes it
probable, as has been said, that the Apostle’s father may have been
among them. (Ellicott also points out that at this time in history it
was the Emperor's prerogative to make someone a citizen)
From Gill's:
Paul said, but I was free born; being born at Tarsus; which, as Pliny
says, was a free city, and which had its freedom given it by Mark
Antony, and which was before the birth of Paul; and therefore his
parents being of this city, and free, he was born so.
(Note that the significance of Tarsus being a free city is disputed)
In either case, it is very likely that Paul's father was a citizen. We are not told how one of Paul's ancestors obtained this privilege. Per commentaries by Jamieson-Fausset-Brown & by Meyer, possibilities include:
- Some special service to the empire
- Purchase (perhaps legal, perhaps by bribing an official)