John Calvin Commentary Acts 22:26

"And when the centurion heard it, he went to the chief captain and told him, saying, What art thou about to do? for this man is a Roman." — Acts 22:26 (ASV)

This man is a Roman. Some might marvel that the official in charge of examining Paul so readily believed him, affirming it as if he knew it to be true. For if he were to simply accept Paul’s word, every wrongdoer could, by this device, have escaped punishment.

But this was their customary procedure: anyone who claimed to be a Roman citizen was punished, unless he could produce witnesses who knew him or prove his claim legally, for it was a capital offense for anyone to falsely claim citizenship.

Therefore, the centurion, doubting Paul's claim, referred the matter to the chief captain; and he (as we have said) immediately investigated the matter more thoroughly. And although Luke does not state by what testimonies Paul proved himself to be a Roman citizen, undoubtedly, the chief captain confirmed the truth of the matter before he released him.