Charles Spurgeon Commentary Acts 25:6-7

"And when he had tarried among them not more than eight or ten days, he went down unto Caesarea; and on the morrow he sat on the judgment-seat, and commanded Paul to be brought. And when he was come, the Jews that had come down from Jerusalem stood round about him, bringing against him many and grievous charges which they could not prove;" — Acts 25:6-7 (ASV)

It was easy for them to make many and grievous complaints against Paul, yet it was not only difficult but impossible for them to prove their charge against the apostle.