Rev. 2:9b I know the blasphemy of them which say they are Jews, and are not, but are the synagogue of Satan.
Rev. 3:9 Behold, I will make them of the synagogue of Satan, which say they are Jews, and are not, but do lie; behold, I will make them to come and worship before thy feet, and to know that I have loved thee.
What do we know of the synagogue of Satan? As with the Nicolaitanes, there is plenty of supposition. John Gill believes they "were false Christians, nominal professors, and shunned persecution for the Gospel." Matthew Henry says that "the devil has his synagogue...Those assemblies which are set up in opposition to the truths of the gospel, and which promote and propagate damnable errors..." H. A. Ironside thought it referred "to the Judaizing movement that came into the church in the early centuries." In the IVP New Testament Commentary Series, J. Ramsey Michaels wrote "that the synagogue of Satan consisted of Gentile Christians who had Judaized, that is, who adopted Jewish ways or even converted to Judaism, perhaps in order to avoid persecution by the Romans. These represent some of the more "sane" interpretations. (I ran across some really wild ones scattered across the internet.)
The specific term "synagogue of Satan" is used on two times in the Bible, both in the book of Revelation. From those passages we are able to learn:
1. The synagogue of Satan is an identifiable people known to the churches in Smyrna and Philadelphia.
2. The synagogue of Satan say they are Jews but are not.
3. The synagogue of Satan are liars.
4. The synagogue of Satan are blasphemers.
5. The synagogue of Satan were enemies of the churches.
6. The synagogue of Satan will be judged, will worship God in the presence of the Philadelphians, and will know whom God loves.
A good many take the synagogue of Satan to mean literal Jews in the flesh, thinking there may be some comparison with John 8:33-45, where Jesus speaks of those who take pride in being Abraham's seed. He says they really are of their father the devil. Also compare verses such as Rom. 2:28-29; Rom. 9:6-8; Matt. 3:9; etc. This could be in view, as Paul demonstrates "he is a Jew, which is one inwardly; and circumcision is that of the heart." But this is a theological and spiritual understanding held by the churches of God. Why would some Jews in the flesh outside the church claim to be spiritual Jews when they would have no reference point for such a claim? On the other hand, Gentiles who had passed through the churches and then embraced a sort of Judaism would have more reason to "claim" they are Jews, though they are not.
Regardless, the churches at Smyrna and Philadelphia had real enemies who made a false Jewish claims. These synagogues (assemblies) belonged to Satan and did his work. They opposed the churches of God. God knows what will come and will be with His people. In Smyrna He commands them to endure persecution, trials and tribulations. Be faithful even to the point of death. He who commands this is He who was dead, and is alive. Fear not! The church in Philadelphia must also suffer tribulation, but God has set an open door that no man can close. No not even those of the synagogue of Satan can do so. In the end these false assemblies will be humbled, and know whom Christ has loved and whom He has not.