Rev. 2:8-11 (ESV)
"And to the angel of the church in Smyrna write: 'The words of the first and the last, who died and came to life.
 " 'I know your tribulation and your poverty (but you are rich) and the slander of those who say that they are Jews and are not, but are a synagogue of Satan.  Do not fear what you are about to suffer. Behold, the devil is about to throw some of you into prison, that you may be tested, and for ten days you will have tribulation. Be faithful unto death, and I will give you the crown of life.  He who has an ear, let him hear what the Spirit says to the churches. The one who conquers will not be hurt by the second death.'
Synagogue of Satan. Wow! Pretty harsh language, but let us not make apologies for it. What is happening here is that the Jews are beginning to persecute the Christians. At this time during history Christians were regarded by many as being a Jewish sect. However most Jews did not see them as a sect of theirs but a completely different religion, they were right. The two had separated that far. But it was those who refused Christ who had abandoned the faith of their fathers. These people actively persecuted the Christians and would turn Christians over to the authorities any chance they got. In rejecting Jesus as the Christ, they no longer follow the teachings of Moses, or the Faith in the Almighty, but have come to worship a different God.
One of these persecutions is about to begin. Some will be thrown into jail. But remaining faithful to the end, the second death will not hurt them.