When Paul and Silas were preaching in Thessalonica, they dealt with opposition. Their ministry, however, still produced converts, and there were plenty of people in that city who disliked that immensely. In an effort to discredit Paul and Silas, their critics worked diligently to slander them. I'm assuming that they thought they could discredit the message of the gospel if they were able to cast doubt on the messengers of the Lord who came proclaiming it.