Take Timothy as a case in point. He didn't even have one generation of separation and his mother was a Jewess, but he still had to be circumcised by Paul in order to be considered Jewish.
Ergo, someone whose family hasn't practiced brit milah or keeping the Torah may have a Jewish ancestor, but can't claim to be Jewish himself unless he deliberately converts. Basically, he's counted as a Gentile until he decides to take the steps to become (again) a Jew, just like someone without a speck of Jewish blood.
For those claiming that 2700 years ago their ancestors were Israelites, but idolaters (another crime worthy of karat) who ceased to practice Torah at all once they were taken and scattered by the Assyrians, it wouldn't matter even if it were true for the above reason.
None of the above is to say that if someone is a Gentile they can't practice Torah, only that there is a perfectly reasonable Scriptural case that they are no more obligated to certain sections pertaining only to Israelites than they are to those sections pertaining only to the priests. And if a Christian who is seeking God's will and striving to keep His Word makes that case to me in regards to kashrut or the Mo'edim, I have no reason to break fellowship with him or boast over him. He is still serving our Lord to the best of his ability and knowledge.
Shalom
>> “Take Timothy as a case in point. He didn’t even have one generation of separation and his mother was a Jewess, but he still had to be circumcised by Paul in order to be considered Jewish.” <<
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That’s not exactly what the word says.
Paul circumcised Timothy to silence the carping of some of the Jews.