There's been a lot of intermarriage between Jews and gentiles over the past two thousand years. Lots of conversions to Judaism. Lots of Jews leaving the faith.
During Hellenistic times, Jews aggressively sought converts among the gentiles. And they often succeeded. Converts to Judaism are called prosletytes in the New Testament.
Jesus said (Matthew 23:15): "Woe to you, teachers of the law and Pharisees, you hypocrites! You travel over land and sea to win a single convert, and when you have succeeded, you make them twice as much a child of hell as you are."
As he traveled through Asia Minor and Greece, Paul often competed with Jews for converts.
So modern Jews have at best a trace of original Hebrew.
In the 1990s, I read of a genetic study (in Chronicles of Culture) that the average Russian Jew has 5% North African markers, which Russian Slavs do not have. Assuming this North African heritage is Hebrew, it confirms the claim of Russian Jews to Hebrew origins. But it also means they're 95% gentile.
Thank you very much for this answer. Sheds a lot of light on the subject, at least for me. Again thank you for taking the time to answer my question. My God bless you, and yours.