Charles Spurgeon Commentary Acts 28:21-22

"And they said unto him, We neither received letters from Judaea concerning thee, nor did any of the brethren come hither and report or speak any harm of thee. But we desire to hear of thee what thou thinkest: for as concerning this sect, it is known to us that everywhere it is spoken against." — Acts 28:21-22 (ASV)

Although people did not understand it, they spoke against it. This is often a blessing. This is the kind of advertisement that helps the gospel, for if people will only be sufficiently interested in it to speak against it, they will be likely to come and hear it, and some of them will be almost certain to receive it. The truth never spreads so fast as when people oppose it.