Revelation 2:9 ‘I know your tribulation and poverty—but you are rich—and the blasphemy by those who call themselves Jews and really are not, but they are a synagogue of Satan.
Revelation 3:9 Look! I will make those from the synagogue of Satan who say they are Jews yet are not, but are lying—look! I will make them come and bow before your feet and make them know that I have loved you.
In fact it's in part how we know about them.