The phrase “synagogue of Satan” can be ambiguous yet gratuitously used without any clear explanation of what it means. There are two scriptures I’ll use as the basis for my explanation:
“He said unto them, An enemy hath done this. The servants said unto him, Wilt thou then that we go and gather them up? But he said, Nay; lest while ye gather up the tares, ye root up also the wheat with them. Let both grow together until the harvest: and in the time of harvest I will say to the reapers, Gather ye together first the tares, and bind them in bundles to burn them: but gather the wheat into my barn.” Matthew 13:28-30
“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.” Revelation 2:9
It is clear from the parable of the wheat and the tare that the saints of God and the wicked are in the same world together waiting for the harvest of God. But when you look at the synagogue of Satan mentioned in Revelation, it is also clear that this concept is being described on a micro-level within the church itself. In this internet age, many people have been introduced to a myriad of bible-based doctrines (whether right or wrong) and there are a seemingly unlimited amount of interpretations.
One that I hear many use is that the synagogue of Satan in the text is referring to people who call themselves Jews but ethnically they are not. The situation here is that in the early church, for a few decades, there was a majority Israelite population in the church and Gentiles were slowly joining as the Holy Spirit used Paul and Barnabas to open up the floodgates of salvation to the world outside of the ethnic Jews. As written in Paul’s letter to the Romans, a Jew is not one outwardly but inwardly and circumcision is not that of the flesh but of the heart.
What the apostle wasn’t saying was that if someone is ethnically Jew they are no longer that. He was simply saying that what the Jew is really supposed to represent as being the people of God is deeper than just being part of the ethnic group and literal circumcision of the male flesh; but the truest form of circumcision is the cutting away of the fleshly ways of the mind.
So if you take all of this into consideration, Paul was going deeper than saying there are people in the Israelite nation who are imposters. He was saying that he understood that you have those in the church who call themselves the people of God but really are members of another body, and that is the body of Satan.
Now what does that mean? I believe it means that even though you may be in the church and baptized, and on the outside you may seem to be a pillar of good, inside you are totally displeasing to God just as Jesus said about the Pharisees who outwardly appeared beautiful to men but inwardly were ravening wolves.
The synagogue of Satan does not occupy separate buildings, they do not have a different bible, and they are known by the title of Christians. So let us not be asleep but wide awake in the spirit because as Christ said, “many shall say to me in that day Lord, Lord…” Y’all know the rest…