Scripture makes clear that not everyone who is descended from the Jews is a Jew. Rather, a Jew is someone who, by faith, is in right relationship with God.
My argument is based entirely on Scripture, and not at all on 'universal church' dogma.
Yes, we are grafted in as believers. In that context this could mean those were non-believers as well (and they were- my opinion). But Revelation was almost certainly written prior to the destruction of the Temple, so at best Gentiles were just starting to show up in significant numbers. So this was addressing primarily Jewish believers. Gentile believers would have been accepted as converts, but converts need a lot of teaching to get up to snuff. Jewish believers would simply have changed sects to that of the Nazarenes (the one where Paul was said to be a ringleader). All would have accepted Jesus (or Yeshua at the time) as the long awaited Messiah. But some, who apparently didn't claimed they were Jews (according to the text).
Galatians 3:1-3 ESV 3 O foolish Galatians! Who has bewitched you? It was before your eyes that Jesus Christ was publicly portrayed as crucified. 2 Let me ask you only this: Did you receive the Spirit by works of the law or by hearing with faith? 3 Are you so foolish? Having begun by the Spirit, are you now being perfected by the flesh?
Apparently there were those preying on congregations of mostly Gentile converts, telling them that they had to keep the "law" before they could be saved! They preyed on these because most Jews knew better. No one keeps the law perfectly, however with repentance comes grace. It has always been that way, otherwise it all would have ended with Adam. Or Abraham if you like. Or Moses. Or any of us. So any way you slice it, the plain text reading stands. Those claiming to be Jews who are not.