The Lord mentions slander from “those who say that they are Jews but are not.” It would appear that (some from) the Jewish population of the city rejected the gospel of Jesus Christ as the fulfillment of Old Testament Scriptures and now turned on those who accepted Christ as Scripture’s fulfillment.
9 “‘I know your tribulation and your poverty ( but you are rich) and the slander of those who say that they are Jews and are not, but are a synagogue of Satan.