John Calvin Commentary Acts 23:25

"And he wrote a letter after this form:" — Acts 23:25 (ASV)

And he wrote a letter. First, we must briefly inform readers who are not familiar with history that this Felix was the brother of Pallas, who, as Caesar’s freedman, became equal in wealth and power to the leading figures of the city. Furthermore, the senate granted him the honors of a praetor, accompanied by titles of disgusting and shameful flattery. Therefore, since the servants of Claudius, abusing his foolishness, ruled the Roman Empire as they pleased—especially Narcissus and Pallas—it is no wonder that Pallas appointed his brother as governor of Judea.

The main purpose of the letter is this: that the commander might use it to create a favorable predisposition in Felix towards Paul; that he might inform Felix of the wrongdoings of Paul's adversaries; and that he might thereby discredit them, so they would be unable to do Paul any harm.