christians how do you explain this verse from the book of revelation?

Part of the covenant made between God and his people was conditional on them acknowledging him as their God and being obedient to his laws to be his people. But from the time that God first made that covenant many of the Jews did not adhere to that, and in a spiritual context they are not his people. The book of Revelation is highly symbolic, and that verse is not speaking about all Jews, but those Jews who spiritually are not Jews. That is what it means by them not actually being Jews, those who are physically born Jewish, and claim to be Jews, but whose mindset make them not his people.