Revelations 2:9
To the Church in Smyrna Jesus revealed this through the apostle John:
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.
The saints at Smyrna were being bitterly attacked by the Jews. Historians tell of the eagerness with which these Jews sought to aid in the martyrdom of Polycarp. As Jews, they claimed to be God's chosen people, but their blasphemous behavior showed they were a synagogue of Satan. (Believers Bible)
Commentary
The afflictions of the Smyrnean Christians are likely to have been due to the persecutions they had suffered. (For this see Heb. 10:32-34, and contrast what is said of the Laodiceans in 3:17). The slander of the Jews of Smyrna is characteristic of the Jewish bitterness against Christians in this city, and is referred to by other Christian writers. These Jews would have taken the opportunity to inform against the Christians.
The church of Smyrna later cited the allegations of the Jews who accused Polycarp of resisting the state religion. They accused him of being the teacher of Asia, the father of the Christians, the puller down of our gods, who teaches not to sacrifice nor to worship them. Such Jews were no longer worthy of the name Jew, but had become a synagogue of Satan, compare Numbers 16:3 which reads the synagogue of the Lord in the LXX).
(They gathered together against Moses and Aaron, and said to them, "You take too much upon yourselves, for all the congregation is holy, every one of them, and the LORD is among them. Why then do you exalt yourselves above the synagogue of the LORD?" Num 16:3)
The name Satan means an accuser, slanderer; this group of Jews had approximated to his nature. Naturally this is not an indication of John's view of Jews; he was a Jew himself! It reflects the depths of apostasy to which this congregation had sunk.
(New Bible Commentary 21 Century Edition. Published by Intervarsity Press, Downers Grove, Illinois, USA.)
Background
The Christians of Smyrna knew poverty because they were robbed and fired from jobs in persecution for the gospel. Early Christians joyfully accepted the plundering of their goods, knowing that they had an enduring possession in heaven (Heb_10:34). This kind of economic persecution was one important reason why Christians were poor in Smyrna. Even today, this is a common form of persecution against Christians. (David Guzik)
Jews by national descent, but not spiritually of the true circumcision. The Jews blaspheme Christ as the hanged one. As elsewhere, so at Smyrna they bitterly opposed Christianity; and at Polycarp's martyrdom they joined the heathens in clamouring for his being cast to the lions; and when there was an obstacle to this, they burnt him alive; and with their own hands they carried logs for the pile. (Jamison Fausset Brown)