Revelation 2:8-9 And unto the angel of the church in Smyrna write; These things saith the first and the last, which was dead, and is alive; I know thy works, and tribulation, and poverty, (but thou art rich) and I know the blasphemy of them which say they are Jews, and are not, but are the synagogue of Satan.
This is from Revelation 2:9 and scripture often misused to try and legitimize antisemitism, people saying things like all the Jews in Israel are fake, or all Jews are Satanic. This is a preposterous lie. As to the scripture, in Christ's time there were persecuting and blaspheming Jews, even in the face of Christ, and Christ told this bunch they are of their father, the devil. But this was not all Jews, and this was not to say the Jews of the synagogue of Satan were not hereditary and practicing Jews, in fact, but of evil hearts of Satan, not recognizing their own God and His own church, like the hateful Scribes and Pharisees that sorely persecuted Christians. The early church had very hard times, from Jewish and gentile opposition. And apparently, in Smyrna, there was a synagogue and example of these hateful, persecuting Jews.