John Calvin Commentary Acts 22:28

"And the chief captain answered, With a great sum obtained I this citizenship. And Paul said, But I am [a Roman] born." — Acts 22:28 (ASV)

With a great sum. The chief captain raises this objection to refute him, as if to say that freedom of the city is not so common and easily obtained. How can it be that you, being some lowly fellow from the country of Cilicia, could obtain this honor, for which I paid dearly?

When Paul answers that he was free born—even though he had never seen the city, and indeed, his father perhaps had never been there—this should not trouble anyone. For those skilled in Roman history know that certain people who lived in the provinces were made free of the city. This could happen if, having served the commonwealth well, either in war or in other important affairs, they desired and requested this reward from the deputies (proconsuls). Therefore, it is not absurd to say that he was born a citizen of Rome, even though he was descended from ancestors from a province far distant from Rome and had never set foot in Italy.

Nevertheless, the question is how it is consistent that the chief captain was afraid because he had bound a Roman citizen, and yet he did not release him from his bonds until the next day. Perhaps he postponed it until the next day, lest he show any sign of fear.

However, I think the chief captain was afraid because Paul was bound at his command to be scourged. This fear arose because such an action meant inflicting injury on the body of a Roman citizen and violating the common liberty—even though it was lawful to put a Roman in prison.