What does Acts 25:18 mean?

Festus is recounting to King Agrippa II the trial he oversaw, suggesting the Sanhedrin intended to accuse Paul of egregious crimes against the state and the Mosaic law. Of course, this is not what happened. In the initial trial, two years before, the Sanhedrin had accused Paul of inciting riots throughout the Roman Empire, defiling a religious structure, and leading a cult (Acts 24:5–6). Under certain circumstances, any of these could be capital offenses—even for a Roman citizen, as Paul was.

Digging into the evidence, neither Felix—the governor then—nor Festus see the crime. Although Paul has been involved in several riots, only two have been in the governors' jurisdiction: on the temple Mount when a mob attacked Paul because of a false accusation (Acts 21:27–36), and a fight between the members of the Sanhedrin (Acts 23:6–10). Paul didn't defile the temple, and his accusers refused to come to either trial (Acts 21:27–29; 24:17–19). And, so far as the Romans are concerned, Christianity is a sect of Judaism, not an illegal religion forbidden by Roman law.

Although it's unclear in the text, the foundational "crime" Paul seems to have committed is insisting that Jesus of Nazareth rose from the dead. The Sanhedrin may disagree, but as far as the Romans are concerned, it's merely "certain points of dispute with him about their own religion" (Acts 25:19).