David Ben Gurion was asked to define "Jew". He said, "Anyone who calls himself a Jew is one. Who would say they were a Jew if they were not?" This was a clever reply, but not true. Jesus warns of people who say they are Jews, but are not. Twice in the book of Revelation, we are warned of them.
Now who would benefit from such a lie? One may think being called a Jew is a great disadvantage. There was a time in a nation's history when an identity had to be chosen. It was either Moslem or Jew.
Judaism was adopted by a non-Jewish people. They had no heritage or connection to Israel in any way. If you have read The Thirteenth Tribe, by Arthur Koestler, this is laid out there. There are Jews, or fake Jews, who think that they are on a path to world domination without their Messiah. My beloved Torah Jews understand this so well and try to warn against it.
Am I anti- Jewish? God forbid. But I am anti-Zionist, where unreligious people seek to gain power before the time for it. Until Jesus returns, Israel will be under gentile domination (Luke 21:24).
These people may not be the ones Jesus warns us against. But the warning is there.
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.