Rev 3:9 Indeed I will make those of the synagogue of Satan, who say they are Jews and are not, but lie—indeed I will make them come and worship before your feet, and to know that I have loved you.
Who are these so-called Jews worshipping in the synagogue of Satan?
Who are the Jews?
Dan 1:8 But Daniel purposed in his heart that he would not defile himself with the portion of the king's delicacies, nor with the wine which he drank; therefore he requested of the chief of the eunuchs that he might not defile himself.
Israelites, who were also Chaldeans, were disciples of Moses and they could not partake of the food generally consumed by the Chaldeans (Babylonians) and Gentiles. Since the Israelites could not assimilate themselves with the rest of the Chaldeans because of restrictions of Judaism in culture (food) and religion, so the other Chaldeans called them Jews.
Dan 3:8,12 Therefore at that time certain Chaldeans came forward and accused the Jews... "There are certain Jews whom you have set over the affairs of the province of Babylon: Shadrach, Meshach, and Abed-Nego; these men, O king, have not paid due regard to you. They do not serve your gods or worship the gold image which you have set up."
Est 8:17 And in every province and city, wherever the king's command and decree came, the Jews had joy and gladness, a feast and a holiday. Then many of the people of the land became Jews, because fear of the Jews fell upon them.
During the time of Queen Esther, many Chaldeans accepted Judaism and became Jews. So Jews are disciples of Moses or followers of Judaism and not just Israelites the descendents of Jacob.
Disciple of Moses were Saved
John 5:45-47 "Do not think that I shall accuse you to the Father; there is one who accuses you—Moses, in whom you trust. For if you believed Moses, you would believe Me; for he wrote about Me. But if you do not believe his writings, how will you believe My words?"
The devout Jews from foreign land, who believed the writings of Moses, sacrificed much just to come back to celebrate Pentecost . So at the preaching of the gospel by Apostles, they accepted Jesus as their Messiah. But hypocrites mocked at the gifts and power of God.
Act 2:5, 8,12-13 And there were dwelling in Jerusalem Jews, devout men, from every nation under heaven... And how is it that we hear, each in our own language in which we were born?.. So they were all amazed and perplexed, saying to one another, "Whatever could this mean?" Others mocking said, "They are full of new wine."
Hypocrites and So-called Jews
Mark 7:6-7 He answered and said to them, "Well did Isaiah prophesy of you hypocrites, as it is written: 'This people honors Me with their lips, but their heart is far from Me. And in vain they worship Me, teaching as doctrines the commandments of men.'"
These hypocrites and heretic Jews, who taught the doctrines of men cannot be classified as Jews (disciples of Moses) but agents of Satan. (Please read Mat 23)
In fact, at the death of Jesus on the cross, Judaism was destroyed and Jews are no longer the people of God but Gentiles just like the Greek, Romans and Barbarians who need the gospel of Jesus for salvation.
Rom 10:11-12 For the Scripture says, "Whoever believes on Him will not be put to shame." For there is no distinction between Jew and Greek, for the same Lord over all is rich to all who call upon Him.