Hellenistic Christian Jews would certainly be considered the lost sheep of the house of Israel. Hellenistic Jews simply means diaspora Jews. The author of Hebrews quoted to his readers the Greek Old Testament, not from the Hebrew, and so that is why the book must have been written to Hellenistic Christian Jews.
>> “Hellenistic Christian Jews would certainly be considered the lost sheep of the house of Israel” <<
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Absolute nonsense!
Jews are the House of Judah.
The diaspora contained no Jews. They were entirely of The House of Israel plus their Levites.
Paul did not use the LXX, that was substituted in for the Hebrew text by the Greek translators after Paul wrote his epistle. The dispersed Israelites all were fully capable of reading Hebrew. They had the scrolls.
The epistle was written to Messianic Hebrews that were still attempting to maintain their traditions afar off from Jerusalem. Hellenists were pagans, as that was the essence of Hellenism, not Christian.
The author of the book must be a Biblical illiterate.