We found out recently that my father is ethnically Jewish. It had been kept secret, but his mother's mother mother, etc. was Jewish. He doesn't consider himself to be Jewish, though. Would I still be permitted to refer to myself as half Jewish if my father doesn't think of himself as Jewish? I don't want to offend anybody.
I would recommend only referring to yourself as Jewish if you are a practicing Jew although the term “practicing” is ambiguous. According to the traditional community, one is Jewish if they have a Jewish mother. In the Reform / Progressive Jewish community, one is Jewish if they were raised Jewish and have at least one Jewish parent. If your father does not consider himself Jewish and you were not raised Jewish, your situation would not meet either criteria as Jews do not use the term ‘half -Jewish’.
It might make sense to refer to yourself using the same phrasing that you mentioned in your ask. “My father does not consider himself to be Jewish, but his mother’s mother was Jewish.”
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