The synagogue of Satan is mentioned twice in Revelation, once in Jesus’ letter to the first-century church in Smyrna and once to the church in Philadelphia. In both cases, the synagogue of Satan is opposed to the mission and message of the church. 

To the church in Smyrna, Jesus says, “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 

To the church of Philadelphia in Asia Minor, Jesus says, “ 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. ” - Revelation 3:9.