Revelation 3: 9-10

 ⁹ I will make those who are of the synagogue of Satan, who claim to be Jews though they are not, but are liars—I will make them come and fall down at your feet and acknowledge that I have loved you. ¹⁰ Since you have kept my command to endure patiently, I will also keep you from the hour of trial that is going to come on the whole world to test the inhabitants of the earth.

I did a search on the reference to the "synagogue of Satan" in verse nine, and it appears that there is no solid knowledge about which strain of Judaism Jesus is referring to in this letter addressed to the Christian members of the church at Philadelphia. Scholars infer that it might likely be a sect Christ regarded as he did the Pharisees, which clung to man-made religious law rather than the laws of God. Other articles I looked at suggested that the 'Synagogue of Satan' may have been a sect trying to impress circumcision on Christians and pressure them to adopt and follow Mosaic law.

 

Revelation 3: 11-13

¹¹ I am coming soon. Hold on to what you have, so that no one will take your crown. ¹² The one who is victorious I will make a pillar in the temple of my God. Never again will they leave it. I will write on them the name of my God and the name of the city of my God, the new Jerusalem, which is coming down out of heaven from my God; and I will also write on them my new name. ¹³ Whoever has ears, let them hear what the Spirit says to the churches.

 

Here Jesus is encouraging his followers in the church of Philadelphia to cleave to their beliefs about Salvation and continue to repent and obey the laws of Christ so that they can keep what He implies are their already steadfast beliefs in God.